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

B. The occasion and the audience

You should always consider who you are talking to and the occasion you are talking for when it comes to formal vs informal. For example a school speech would be a different occasion than than a private conversation and you'd want to speak formally at the first and informally at the latter.