One thing that I always notice when reading British and American novels one after the other is the difference in their use of the Present Perfect tense. As any fellow English language teacher would be able to corroborate, teaching the Present Perfect is the bane of the English as a Foreign Language classroom.
It is a nightmare tense for most foreign speakers, and one of its definitions immediately shows why: “actions completed in the past with an effect on the present”. But it could also be describing an action still in progress. Or an action completed in a period of time that is still in progress. What does that mean in practical terms? It’s always hard to explain, and the different language backgrounds of students do not help at all.
Thus, a key point about American English is the reduced presence of the Present Perfect tense. American English speakers define events that are still connected to the present more narrowly than other English speakers, especially when using the words already, just and yet. This can be seen in the table below:
However, this is not really a hard-and-fast rule; it all depends on how the speaker perceives an event. If for the speaker the event is completed and disconnected from the present, then the past simple will do. If the speaker envisions a continuation or connection with the event to the present, then the automatic choice for both British and American speakers would be the Present Perfect.
Of course, there are also situations when the grammar demands a specific tense: if you still live in a specific place, you would say “I’ve lived here for two years,” while if you don’t live there anymore, the past simple would be the only grammatically correct option for all English speakers.
For most learners of English today, this has really become a matter of personal choice – speakers mix and match different Englishes depending on their taste and the different sources from which they have learnt English. That is not particularly problematic, as long as they are not trying to publish a book or paper in a specific country or sitting for an internationally accredited exam. Then, consistency is key, and writers will have to choose one English variety and stick to it. Editors can often help with this.