It's like grammar: You don't sit around articulating "this is the subject, this is the object, and I need the third person singular over there" when you write -

It's like grammar: You don't sit around articulating "this is the subject, this is the object, and I need the third person singular over there" when you write - but knowing the mechanics of grammar allows you to write texts with correct grammar more or less intuitively, and it's easy to reach back and pull out a rule when you think something sounds weird.