Back when I was a high-school Latin student, we had to memorize "the four principal parts" of every new verb. Just learning amare ("to love") wasn't enough information, but learning the four separate forms amo, amare, amavi, amatus allowed you (in theory) to deduce every single one of the verb's possible conjugational forms, in all tenses, active and passive, subjunctive and imperative, etc.

Russian is a bit simpler than Latin, but generally speaking, for most nouns and verbs there will be 2 to 4 "essential forms" that you need to memorize, not just one form.