what's he going to do when he -----(grow)up?

问题描述:

what's he going to do when he -----(grow)up?
我认为应该是GROWS,可怎么江湖上又流传说填原形,

不是原形.要用第三人称单数grows.
通常助动词(例如do),情态动词(must, should, ...),和不定式to 后面才跟原形.