From Grammar Girl

http://grammar.quickanddirtytips.com

Is "Swum" a Word?

I'm frequently asked a variation of this question from Vivian M.: 

I know that "swam" is the past tense of "swim." Is there such a word as "swum"? It sounds peculiar but I think it's correct, though I never hear anyone using it.

"Swum" is a word; it's the past participle of "swim." You use it when writing in certain tenses.

A trick to help you remember when to use "swum" is that past participles always come after a helping verb such as "have" or "had."

·  I have swum. (present perfect tense)

·  I had swum. (past perfect tense)

·  I will have swum. (future perfect tense)

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