how context influences the way we speak and our choice of vocabulary and grammar?

Answer 1

Well, a lot.
Imagine you're in a job interview or with your mates at the pub.. of course you're much more stiff and careful choosing every word in the first, and much more relaxed and vulgar in the secont ex. no?

Anyway i dunno if i got the point of your question.

Answer 2

I think studying abroad where English is the medium or having to converse with English teachers/friends/colleagues definitely influence the way we speak since we need to learn how to pronounce words, sentences, etc. clearly so that the others can understand what you want or are talking about. As for the choice of vocabulary and grammar, I think, we can acquire more from your conversation and also from our reading in daily newspapers, watching TV programs and mass media around you, etc.