Obviously, * Desu/deshita/de arimasen/de arimasen deshita * Masu/masen/mashita/masen deshita

Then add a few more * if * while * want to <verb> * passive * honorific * etc.

You can learn one a week or one a month. There aren't very many to learn. But having just four or five up your sleeve (in addition to verb conjugation) can really improve your comfort.