'-ei-' or '-ie-'?

Words that have the letters 'e' and 'i' together a common source of spelling mistakes, and there is a commonly known spelling rule about them:

'i' before 'e' except after 'c'

Like many spelling rules, this doesn't give the whole picture and is subject to many exceptions. (You might have a friend or a neighbour named Keith who is a scientist, for example.)

There have been several attempts to expand the rule to cover all exceptions. But ultimately, the only way to learn to spell correctly is to learn the spellings of awkward words.

There are examples of both the rule and its exceptions listed in Category:Spelling common errors.