The verb dekiru (できる) means to be able to or can. It can be very useful especially when put together with other verbs.
(I) can't (speak) Japanese = nihongo dekimasen (日本語できません)
I can't (play) piano = boku wa piano ga dekinai (ピアノができない)
Can you also (speak) french? = furansugo mo dekimasu ka
You can also use this verb in conjunction with other verbs to mean you can or can't do something else.
(I) can dance = dansu suru koto ga dekimasu
(He) can't write katakana = katakana o kaku koto ga dekimasen
Can you drink wine = wain o nomu koto ga dekimasu ka
(I) can't swim! = oyogu koto ga dekinai yo! (泳ぐことができないよ)
(I) can't do that kind of thing = sonna koto dekimasen