There is a big difference between understanding the grammar of a language and being able to speak that language well. You don’t need to understand how a car engine works in order to drive it and you don’t need to understand grammar to speak a language. Knowing grammar will, however, perhaps give you more confidence to speak, as you will be less afraid of making mistakes. Almost every grammar rule has an exception, so the best way to improve your English is to practise as much as you can.