Contractions are used when speaking and when writing in casual situations. In formal writing, they should be avoided. In formal writing, unusual phrasal verbs as well as idiomatic expressions should also be avoided.

Contractions of Question words are normally only used when speaking or quoting what was said, for example, a dialogue in a novel.

Informal contractions should only be used when speaking in very casual situations.

Common informal contractions

am not, is not, are not --> ain't

bet you --> betcha

don't you --> dontcha

give me --> gimme

going to --> gonna

got (have) --> gotta

kind of ---> kinda

let me --> lemme

ought to --> oughta

sort of --> sorta

want to --> wanna

what are you --> whatcha

Informal contractions are contractions that break grammar rules and are only used in very casual speech. Two or even three words could be merged. An apostrophe is often not used.