Being that in Japan you can be paid $50 on up per hour to teach english, with absolutely no ability to speak Japanese.
Something that valuable suggests exposure to language is a key step to learning or improving. In Spain I learned quite a bit of Spanish students students that wanted to practice their english by speaking to me. They get feedback and learn pronunciation first hand.
Conjugation is important. If you want to speak another language to somebody then you will have to know your past, present, future tenses, combined with singular, plural or we us they you are almost always used in any conversation so they are also important.
If you are already fluent in english try using spell/grammar checkers that comes with whatever word processor you use.
You learn from your errors that are found, and from the suggested corrections to these errors.