“TOM must have goodhibits."TOM是第三人称，为什么不能用“has”而用“have”

must是情态动词接原型，其他如may might can could should shall 等都是情态动词

must后面用动词原形

must是情态动词 后面必须用原形
诸如should \can

must = have to, 后面加原形动词

因为前面有个 "must",当动词前有 must, can 等的助动词时均要用动词原形,
must do sth.明白吗?