https://www.languagetransfer.org is an excellent free source for learning grammar. It will get you really far in terms of understanding the rules of a language. From there it's a lot of memorization of vocabulary.
For me, learning Spanish required flash cards (https://calcubest.com/languages/spanish/tenses) to actually memorize which conjugation rule applies to which tense. But Language Transfer was definitely what made the rules click.
For me, learning Spanish required flash cards (https://calcubest.com/languages/spanish/tenses) to actually memorize which conjugation rule applies to which tense. But Language Transfer was definitely what made the rules click.