It's arguably more proper to say "nihongo ga hanasemasen" (I can't speak Japanese) than "nihongo wo hanashimasen" (I don't speak Japanese). The first is a statement of my personal abilities. The second could easily be a statement of my attitude. "nihongo ga hanasemasu kedo nihongo o hanshimasen" (I can speak Japanese but I don't speak Japanese)
For whatever reason I've never heard wakaremasen (can't understand) but only don't understand (wakarimasen).
Well, the original wording was "hanasei masen" and I assumed the OP wouldn't know about the potential form at their current level. Potential form when talking about knowing the language doesn't sound right to me, but I can find example sentences using both potential and normal form. In natural language I think normal form would be used. Potential form of "wakaru" is "wakareru" which happens to be a verb with a completely different meaning "to diverge/separate/divide", so I don't think anyone uses it and it makes little sense anyway.
> I assumed the OP wouldn't know about the potential form at their current level.
You assumed correctly :-) I thought that adding a "sen" suffix to any verb was the only way to negate it. Wakarimas = I understand. Wakarimasen = I don't understand. (Yes, I read Shogun :-) Japanese is apparently much more subtle than I realized.
I have asked my Japanese friends about this and they are usually convinced after discussion that "ga" is the correct particle to use here. But "wo" sounds fine.
I think it is very much like "was/were" in English. "If I was to give you a cookie..." / "If I were to give you a cookie..." "were" is "correct" (as though there is such a thing in English), I think, but "was" sounds fine. "Was" looks pretty abrasive in writing but say it out loud and you'll notice that you hear it a lot.
> For whatever reason I've never heard wakaremasen (can't understand) but only don't understand (wakarimasen).
One of my teachers explained to me that "wakaru" (to understand) already has the implicit meaning of "able to" in the word itself, so you should never use the potential form. Searches[1][2] for "分かる 可能形" seem to agree.
hanasemasen = can't speak
hanashimasen = don't speak
It's arguably more proper to say "nihongo ga hanasemasen" (I can't speak Japanese) than "nihongo wo hanashimasen" (I don't speak Japanese). The first is a statement of my personal abilities. The second could easily be a statement of my attitude. "nihongo ga hanasemasu kedo nihongo o hanshimasen" (I can speak Japanese but I don't speak Japanese)
For whatever reason I've never heard wakaremasen (can't understand) but only don't understand (wakarimasen).