[maxton]

If you wish to undertand and to be understood, I agree that vocabulary is very important.

I assume from the class of errors you made (understand instead of understood; know instead of knowing) that your native language doesn’t have verb tense like Chinese. But it doesn’t matter if these small grammatical errors are made, because native speakers can still understand sentences with flawed grammar. However, if you are missing vocabulary you will have a hard time understanding what other people are saying, and when you speak you end up spending a lot of time trying to describe words you don’t know in terms of words you do know.