[elldoubleyew]

Grammar rules start by learning verb conjugations, these can be learned through tables, although there are some exceptions you will have to learn individually. This is an excellent free tool for practicing those (not mine, just something I've used): https://baileysnyder.com/jconj/

For structural grammar there are a lot of different routes to go about this, MNN teaches these pretty well in my opinion. I tried using bunpro (https://bunpro.jp) with mixed results but I know some people who swear by it.

As for knowing if your understanding of a sentence is correct, if I have doubts at my level I assume that I am likely incorrect. I typically google the part of the sentence that I am unsure about and either look at images or posts that use it in different contexts. Reverso context is also useful for this (https://context.reverso.net/translation/)