Student or Learner
what r the differences betwen American English and GB English in grammar
There aren't many differences, and British and American speakers can usually understand each other perfectly well. One difference is that American English doesn't use the present perfect tense as often as British English, but this doesn't cause any problems. Another one is that although the subjunctive mood has almost died out in both British and American English, it's used a little more often in American English. (Again, this causes no problems.)
Those are the only differences in grammar I can think of. There are also some differences in spelling, and quite a few differences in vocabulary. Again, this doesn't usually cause too many problems, but the differences in vocabulary can sometimes cause some embarrassing situations.
The biggest differences are in idiom and slang. But most British people have picked up quite a good knowledge of American idiom from Hollywood movies; and now that British films are becoming popular in the US, Americans are also getting used to British idiom.