What's the past participle form of the word exit? Is it exit (irregular, like set)? exited? exitted? On one page I found exited but if that's the case why isn't it exitted (double t) like with the word emit - emitted? Is there a rule when the consonant at the end is doubled and when not?
When we have a word ending in a single vowel and then the consonant 't', the consonant is only doubled before suffixes if that syllable is stressed. So when there is no stress we observe just a single 't'. In the following examples the stressed syllables are premarked with an apostrophe:
However if the last syllable is stressed then we will see a doubling of the consonant:
This is just a rule of thumb as there are special rules for certain prefixes, and compound words and loan words from other languages will not necessarily follow the rule.
Edit: Please also see Janus' interesting comment below about loan words with silent 't's below!