one of the things I still find it very difficult is past tense and past participle.

But English is used around the world, and that means it is not a very difficult language!

Japanese and English

You could also say "Japanese vs English", since that means you're comparing and contrasting them (or that they're fighting each other lol)

one of the things I still find it very difficult is past tense and past participle.

So you do not have to worry about which to use, and, if you awere to learn Japanese, you willould find out how easy ithat is.

I replaced "it" with "that" because "that" refers to "not having to worrying about which to use", whereas "it" sounds like it is referring to "learning japanese".

But English is spoken around the world, and that means it ismust not be a very difficult language!

"must not be" or "must be" is used when a guess or conclusion is based on the previous statement, especially when that guess or conclusion goes against the speaker's personal experience. Here are more examples: "I looked everywhere around here for my phone and couldn't find it, but this is the last place I remember having it so it must be here somewhere!" or "The footsteps lead to that shed, so he must be hiding there."