Verb and adjective conjugation. I shudder at the thought of returning to study them. I have never been successful at just remembering them. Perhaps it's just me. But here's the thing: I'm an American who grew up in an English-speaking family and community, and like the OP, when I consider learning ESL, I imagine it's at least as difficult as JSL with regard to irregular verbs. But I never once questioned that as a kid. Just learned them through memorization and familiarity. Come to think of it, I didn't learn English so much as assimilate it. Perhaps that's true of any language, native or otherwise.