It depends on what your purpose is. But in general, you need to know some basic grammar to be able to understand English things like tenses, structure of sentence, and so on (just basic grammar) I didn't learn English grammar through grammar books and i will not do that, cuz they are too boring. So, at the beginning i read short paragraphs with titles which made me interested in. Whenever i had something i didn't understand, i looked it up on internet because at that time, my brain was ready to remember this grammar, so i could remember it easily. That's how i have learnt English grammar: just read, look up and remember. It works for me.

I know .but I don't to do it .my English is bad.

Is English grammar very important? Yes, it is.

一定是!