I don't know what _____（eat） for lunch

I don't know what _____（eat） for lunch
I don't know what _____（eat） for lunch

I don't know what _____（eat） for lunch
to eat

楼主：
I don't know what _to eat____（eat） for lunch

to eat
动词不定式作前面名词What的定语，什么吃的、吃的东西

to eat