It can be a verb, noun, or adjective. Startle, verb - similar to shock, surprise as verbs --> common. Startle, noun - a shock, or a startle --> not very common (he had a startle when I jumped out from under the bed) Startle, adjective --> I was startled when I got my results! Quite common.

I'll answer your question here, as this has nothing to do with the other page where you wrote your comment. What does 'would' mean? Do you mean 'Why am I using a conditional form? I've used it to say that if you wanted to write this sentence in a more normal way, the expression would be 'woke with a start'. Is your textbook a Chinese one? Sometimes Chinese textbooks and websites have some rather strange English.

This sounds very strange to me. I have never heard of 'startle' being used as a noun. The normal expression here would be 'woke with a start'. Where did you see this sentence?