“Not long after the American Declaration of Independence, Noah Webster had reasons to believe that British and American English would in the long run drift apart: ‘several circumstances render a future separation of the American tongue from the English, necessary and unavoidable’ (Webster 1789: 22). These expectations have not been confirmed, and there are at present no signs that this will happen even in the distant future. ”(G. Rohdenburg, J Schluter, One Language, Two Grammars?:01)
Although we talk about the same language, English, we notice that it is slowly evolving into two different languages, with two different grammars. On one hand the grammar of British English, which tends to be more conservative and less open to accepting new rules or to changing the old ones, and on the other hand the grammar of American English, which, on the contrary, is tolerable concerning changes, yet exceptions may always occur. In order to underline the differences between British and American English grammar, we will talk mainly about the verb, especially the irregular verbs and the way in which the verbal tenses and the moods are used in both languages.

Regarding irregular verbs, unlike British English, American English is an amazing mixture of innovative and conservative tendencies. We noticedthat in the cases in which the past tense, as well as the past participle, ends with a -t, the final -t disappears and an –ed form is added, yet this occurs only in American English, in British English verbs keep their irregular form. For instance, if in British English we have the forms bust-bust-bust, kneel-knelt-knelt, leap-leapt-leapt, in American English we have bust-busted-busted, kneel-kneeled-kneeled, leap-leaped-leaped. This tendency of regularization of irregular past tense and past participle can also be noticed in verbs like: burn-burned, lean-leaned, or spell-spelled.

Rohdenburg and Schluter stated in their book “One Language, Two Grammars?” that regular verb forms were frequently used in British English before they started spreading in American English; obviously the number of irregular forms in the second half of the nineteenth century and the first half of the twentieth century, was considerably smaller than the number of regular forms. “In the second half of the twentieth century, irregular verb forms gain ground again in British English. It may well be the case that the currently more conservative nature of British English with respect to this variable has to be attributed to an avoidance strategy treating the regular forms as a morphological Americanism. American English initially lagged behind British English in this ongoing trend towardsregularization of irregular verb forms; from the second half of the nineteenth century, however, it has been clearly in the vanguard of change.” (G. Rohdenburg, J Schluter, One Language, Two Grammars?:25)
As we can see from the table below, in the eighteenth century American authors had a tendency to use the irregular form more often that the regular form, while, on the other hand, British authors used less irregular forms, displaying a visible preference for the regular ones. However, in the nineteenth century, preferences change, as we now notice a decrease in the usage of irregular forms regarding American writers, while British writers become more interested in irregular forms.

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It is also worth talking about the past participle of the verbs “get”, which is “got” in British English and also “gotten” in American English; and “prove” which is regular in British English while the American English uses the form “proven”. Firstly we will discuss about the form “got/gotten”. While some people claim that the form “gotten” is somehow an inheritance from the old British language, in other words a colonial lag, as in the spontaneous spoken conversations some instances of “gotten “ are still used in the North of England; from text books we find out that there is a slight difference in meaning between the two terms. In American English the form “gotten” has themeaning, to obtain/acquire, while the form “got” means to have/posses. We should also note that while in British English the form “had got to” is used to express obligation, American English prefers the form “had to”, rather than “had got to”. Therefore, we may perceive the morphological Americanism “gotten” as a less used, colloquial variant of got, which is gaining more and more ground in present day American English, written and spoken.
Furthermore, the next form that we will discuss is “proved/proven”. The case of the term “proven” is different of that of the form discussed above, as the en-participle is considered to be an innovation brought by the Scots in the sixteenth century. As we notice in the case of “gotten”, the use of the suffixed past participle has been gaining ground in American English; but, unlike “gotten”, “proven” is not a low frequency and stylistically marked option. Another possible difference between the two forms is that “proven” is not a morphological Americanism, as it has been slowly gaining ground in British English. At this point we can state that “the use of “proven” in American English is not a genuine case of colonial lag but an instance of postcolonial revival”(G. Rohdenburg, J Schluter, One Language, Two Grammars?:24).
Regarding weak past participles, we might say that there is a considerable variation, whereas afamiliar instance of the American-British contrast is represented by the group: burn, dream, dwell, kneel, lean, leap, learn, smell, spell, spill and spoil. Yet, there is a difference, which is frequently less taken into account, meaning the participial forms of the verbs “light” and “knit”. Originally formed with the regular -ed(e) suffix, their participial form evolved through time, thus becoming irregular and giving rise to a monosyllabic form. Therefore, both the regularly formed participial forms (lighted, knitted) and the irregular forms (lit, knit) were allowed and although, since the beginning of the eighteenth century there was a tendency towards replacing lighted with lit and knitted with knit, the two forms remain in conflict to the present day.
Moreover, we will now have in sight the verbal tenses and moods, especially the differences that appear between British and American English. We notice that “Very often a British form, which fell into disuse long ago or may still be heard in a dialect or in substandard speech, is fully accepted as best American usage.”(Iarovici Edith, A History of the English Language; 57 ), for instance, only informal British as well as a small amount of dialects use verbs such as “help” without the particle “to”, while this use of short infinitive is perfect literary standard in American English. Therefore we say
Thismedicine will help cure your illness. (In American English)
This medicine will help to cure your illness. (In British English)
Another difference that we must mention, if we talk about the Short Infinitive, represents its usage in American English in structures like
“Look at him draw.; Listen to him sing.”,
while British English does not allow such constructions, preferring the –ing form or other constructions instead:
Look at him drawing; Look how he runs.
We may add that, regarding the Past Simple, it is used in order to give news in American English and, in addition, it is also allowed to use indefinite past time adverbs such as already, yet, ever, just and before with the Simple Past. On the other hand, in British English we use the Present Perfect Tense with indefinite time adverbs. For instance, in American English we will say
He just went home; or He has just gone home.
While British English considers only the second sentence as being correct.
Furthermore, speakers of American English generally tend to use the Present Perfect Tense, meaning the form have/has + past participle, far less than speakers of British English. In spoken American English it is very common to use the simple past tense as an alternative in situations where the present perfect would usually be preferred in British English.In order to strengthen the statement from above we will have in sight the following situation: when talking about an action from the past that has consequences in the present, American speakers would say
Jenny feels ill. She ate too much
I can't find my keys. Did you see them anywhere?
Instead, British language users would say
Jenny feels ill. She's eaten too much
I can't find my keys. Have you seen them anywhere?
Traditionally, British English uses the Present Perfect when talking about a recently completed action along with the adverbs already, just and yet, while regarding American English, these meanings can be expressed with the Present Perfect, when we want to express a fact, or Past Simple in order to imply an expectation. This rule was adopted quite recently, about 20 or 30 years ago, and it is continuously spreading, yet the British style is still commonly used as well.
'I've just arrived home.' / 'I just arrived home.'
'I've already eaten.' / 'I already ate.'
In British English, the forms “have got/have” can be used in order to express possession and the forms “have got to/have to” can be used for the modal of necessity. We note that the forms that include the verb “got” are more informal, while the ones that are used without the verb “got” are used in formal contexts. Moreover, in American speech theform without “got” is more frequently used than in the UK language, although the form with “got” is often used for emphasis. Colloquial American English informally uses “got” as a verb meaning necessity and also possession, for example: 
I got two cars;  I got to go.
The subjunctive mood, which is morphologically identical with the bare infinitive, is regularly used in American English in mandative clauses, for instance
They suggested that he apply for the job.
In British English, this construction is no longer used since the 20th century, being replaced by constructions such as
They suggested that he should apply for the job.
Another possible similar construction, yet much more ambiguous than the previous one, is represented by
They suggested that he applied for the job.
Therefore, we can state that “In American English the Subjunctive has been preserved to a greater extent than in British English.” (Iarovici Edith, A History of the English Language; . So, we may consider this situation as an exception from the statement presented in the introduction, in which it was said that the British language is more rigid regarding its grammatical rules and that the American English was more receptive to grammatical changes.
Another phenomena worth mentioning in colloquial American English is representedby the speaker’s tendency to simplify the language, while even informal British English does not accept the idea of changing for the sake of simplicity. Spoken American English is much more oriented towards contractions, as the forms “wanna”, “gonna”, “ain’t” are frequently used instead of the forms “want to” , “going to”, or “am/are not” yet these forms are only used in informal American English. Likewise, the use of abbreviations in everyday American language is also very common. For instance, Americans have a tendency of replacing structures such as “Do not disturb”, “Be right back”, “Not Available” with their initials, and so we can just say DND, BRB, N/A and be understood without difficulty. While in British English this phenomena occurs rarely or it is inexistent.
In conclusion, there are many differences in detailed aspects in the use of daily British and American English, they are similar to each other in most of aspects. Therefore, they shall only be considered as different forms of the same language rather than two different languages. In addition, we cannot say which one is better or advanced. On the other hand we can say that the American Language, unlike the British one which is more sensitive concerning grammatical errors, is less rigid and strict, allowing the appearance and usage of new words and adopting new rules just for the sake of simplicity.