Which is correct: "rebuild" or "rebuilt"?

Both rebuild and rebuilt are correct, but they are different forms of same verb used in different grammatical contexts. Rebuild is base form and present tense, meaning to build again. For example, We need to rebuild the house, or They rebuild homes after storms. It is also used as infinitive and imperative, Rebuild this wall. Rebuilt is past tense and past participle, meaning already built again. For example, They rebuilt the house last year, or The house has been rebuilt. So correct choice depends on time and sentence structure. Use rebuild for present, future, or infinitive situations, I will rebuild, we must rebuild, they plan to rebuild. Use rebuilt for actions completed in past, He rebuilt the fence yesterday, or as adjective describing something that has been rebuilt, a rebuilt engine. Both are spelled correctly, neither is inherently more correct, grammar determines which fits. If you are asking which spelling is correct, both are, but usage must match tense and sentence construction.