Having a knowledge of English grammar rules will not ensure success at learning another language. We speak our own native language intuitively, and most people are able to do so without knowing a single grammar rule.
If we had to know grammar rules in order to speak our language, we wouldn't be able to speak it at all. Babies do not study grammar to learn how to speak; they listen and repeat. The same needs to be done in order to learn a foreign language: Learn it by listening to common phrases and repeating them.
Likewise, a speaker of a foreign language does not need to understand their own grammar rules in order to speak their own language, and neither do we. Again, it is listening, speaking, and immersing oneself into a language that makes it easiest to learn a language.