I've been reading poetry lately and sometimes, the meter just doesn't make sense. Now, I know that elision is a thing, but I don't know the rules. When can I elide letters? Are there any hard-and-fast rules for when elision is required, or is it all subjective and dependent on the specific poem?
Elision of a vowel (or vowel + m) occurs when it's at the end of the word, and the next word starts with either a vowel or h + vowel.
At the discretion of the poet, the last vowel of a word can be kept even if the next starts with one; in this case, it's considered to be in hiatus. However, this is very, very rare.
However, if the second word is a form of sum, its first letter is normally elided, instead of the last letter of the preceding word. This is called prodelision.
Thanks to @Cerberus and @Nick for pointing out and naming prodelision.