I’ve seen a lot of my non-native friends struggle with certain verb conjugations, usually the irregular ones.

The hodgepodge rules and patterns in the language

Definitely pronunciation-spelling which really comes down to years of listening amd seeing the texts before one gets evenva little bit comfortable with it. In my area the second biggest scare of English is the strict and to us fairly alien word order. But that's sharing the spot with irregular verbs and perfect tense.

Now that Gabriel mentioned it, I'm starting to think it's a pronunciation thing too... I don't mind too much about my accent because we're in the age of Worldwide English, but that's a different matter. In the movie "My Fair Lady," it was depicted that even modern English people don't pronouncecorrectly, and in "Singin' in the Rain," it was depicted that even modern Americans don't it as well

Yep, pronunciation is basically guessing with English. It's so bad that even natives make mistakes regularly.

Pronunciation is most difficult for me. There are a lot of phrase that front and back words is connected.

Pronunciation

depends on what their native language is as for me, probably prepositions