Tobject]user: two things to keep in mind when you're deciding whether to use formal or informal english are the
a. opinion and the occasion.
b. occasion and the audience.
c. reader and the location.
d. reader and the audience.

Two things to keep in mind when you're deciding whether to use formal or informal English are the B) occasion and the audience.
Of course you are going to change the way you talk depending on whether you are speaking to your friends or to your teacher - usually with the teacher you are going to use more formal English.
Occasion is also important - you won't speak in the same manner at a party and a cocktail.