Basic principle: most of the verb forms are contracted, which is one of the reasons why they are so hard to remember. The rules themselves are really simple, but the contractions are not. First point: all verbs have 5 basic conjugations that correspond to the five vowels (for “u” verbs). Let’s take 飲む as an example:
飲む (sentence ending/adjectival form) – 私は飲む (I drink), 飲む人 (the person who drinks)
飲ま (imperfective: indicates the verbs action either hasn’t started or at the least hasn’t finished) – (飲まない (I don’t/won’t drink)
飲み (conjuctive: turns the verb into a noun and allows it to attach to other words) 飲み放題 (all you can drink)
飲め (potential: indicates possibility or ability) 飲める (I can drink)
飲も (volitional: indicates intention or will) 飲もう (Let’s drink/I shall drink)
Ru verbs are easier, because they all just drop “る” and leave it at that
All of the above are pretty straightforward, except 1: the conjuctive form. This is because in Japanese, the “i” sound is often contracted. This occurs when conjugating the te-from, the past tense, and a whole other slew of seemingly complicated conjugations. Let’s look.
Te-form: 飲みて is the original form (you’ll see it in really old books). However, the “i” contracts and the voiced “m” sound is transferred to “te” to make it “de” --> 飲んで
Past tense: 飲みた is the original form. Contracted, it’s 飲んだ.
There are a whole host of other rules depending on the consonant. If you’re interested to learn about them, let me know and I’ll write more. In general, just remember that when it gets weird, it’s usually because of contractions that no one bothered to explain.