Correct spelling, explanation: the word hasn’t changed much since its Latin origin when it had the form comparare and it had the meaning of to compare or to liken, which is similar to the contemporary meaning. Today it is a common, regular verb, which means that you simply add a -d ending to the base form when you want to create the Past Simple form of the verb, which results in the form compared.

Definition of compare:
1. verb (past form) – to search for the differences or similarities between two things,
He compared two offers he got and rejected the first one.
2. verb (past form) – to suggest that two things are similar or equal,
You often compared the twin sisters – they only looked similar, but they were completely different in terms of personality.

Expressions and idioms with compare:
1. compared apples to/with oranges – to state that two things are so different that there is no point in comparing them,
You can’t be serious that you compared this flat to the one we saw yesterday. You compared apples to oranges!
2. compared notes – to tell one another about something you both tried,
He was also my boss for some time so we compared notes when we met.

Incorrect spelling, explanation: due to similar pronunciation users of English sometimes mistakenly think that the word compared has got something to do with the word pair, which results in the incorrect past form compaired. The form, however, is wrong and the only correct spelling is compared in the past tense.