Grammar is more of a polish than a primer. Most likely you did not learn the grammar rules of your native language until you were about eleven or twelve years old. You were already a fluent speaker.
Grammar's Role in Language Learning
Many language learners have difficulty when presented with verb conjugations and noun declensions in classes taught using the grammar-translation method. Sometimes the students don't know the difference between a verb and an adverb. Memorizing "je suis, tu es, il est" is of less help in comprehending French conversation than learning a dialogue where the verb "to be" is used naturally. However, once the student has progressed to the stage of composing essays, especially if this is for an advanced college course, then grammar rules will help eliminate basic errors.
Do You Know the Grammar Rules in Your Own Language?
Unfortunately, English speakers have been riding a pendulum of language learning theories that have almost corrupted the language. First, a few generations back students were misled to believe that ending sentences with prepositions and splitting infinitives would be committing unforgivable grammatical sins.
Then, Sir Winston Churchill chided an editor when he was corrected for committing the first "sin." Churchill asserted that changing his manuscript to correct it was "the sort of thing up with which I will not put." Demonstrating that any native speaker of English, no matter how well-educated, would most likely have ended that sentence not with just one preposition, but with two of them.
As for splitting infinitives, the long-running Star Trek series regularly introduced the show with the most famous split infinitive of them all with the phrase: "to boldly go where....". Now, grammar is not taught at all.
What Happens When Grammar is Ignored Altogether?
No English speaker would ever say "Us went to the store" or "He gave it to we." Yet frequently even teachers and television reporters are heard to say something along the line of "John and me went to the store" or "He gave it to John and I."
The second set of sentences is every bit as outrageously ungrammatical as the first set is. It's a matter of case and number, not formality or politeness or whatever rules are being dreamt up for the first set. "We" and "I" are the subjective case of the first person pronouns; while "us" and "me" are the objective case for the same pronouns.
The difficulty English speakers have with this is that we have only three cases in English, and they are used only with pronouns. Russian and Ukrainian, for example have five or even six cases, and use them with pronouns and nouns and adjectives. They would never make the case confusion errors English speakers make. For this reason, if English speakers want to learn Russian, it would help if they first learned basic English grammar.
What Use is Grammar?
Language is like sport. If you know the rules, then you can break them with impunity. Sometimes.