For Imagine Past Tense?

The Past tense of imagine is imagined.

Is Imagine if correct?

(1) Imagine that you are a slave. (2) Imagine if you are/were a slave. (1) is strictly correct in formal or written English. (2) is colloquially acceptable (comparable - and perhaps confused - with "What if...?"), but probably to be avoided in writing.

Is imaginate a verb?

verb (used with object), im·ag·ined, im·ag·in·ing. to form a mental image of (something not actually present to the senses). to think, believe, or fancy: He imagined the house was haunted.