which one is the right grammar? 我们什么时候说 “我没吃完” “我吃不完” “我不吃完”
those are all sentences that make sense, the right grammar depends on what you want to say! the first and the last means you havent finished eating. 没 (mei) denoted that it is in the past (maybe the plate sat there for a while but you havent finished yet) while 不 (bu) denotes that it's ongoing or future. (maybe you are currently eating and someone asks if you are coming with them and you tell them you havent finished eating, like "cant you see, i'm not finished eating!")
and the middle one means "i cant eat it all" (as in i cant finish this meal. i will eat not finished)
here is also a good resource for the difference between bu and mei:
dont know how i can make the url capture the whole thing, it cuts off at the ". sorry about that
“我没吃完” = I didn't eat it all “我吃不完” = I can't eat it all “我不吃完” = I don't eat it all (doesn't want to in most of cases)
“我没吃完” - I didn't finish eating it. “我吃不完” - I can't finish eating it. “我不吃完” - I'm not gonna finish eating it.