denotes everything except the thing listed, roughly translating to "anything but". It is the antonym of だけ. It can only be used in the negative to form a double negative, meaning that しか〜ない again has the same meaning as だけ.
勉強するしかない To have nothing to do except to study
魚しか食べなかった I did not eat anything but fish
日本語しか使わない I don't use anything but japanese (I only use japanese)
No choice but to (...)
Similar to must (not), denotes that there is nothing else you can do but (...) Very much in line with main meaning, but very common structure.
もう電車が来ないから、歩くしかない。 Since the trains dont come anymore, there is no other option but to walk.
今月は休めないから、コンサートに行くのを{我慢|がまん}するしかない I can't take a day off this month, so I have no choice but to resist going to the concert.
sometimes slang-ified in manga and tv to っきゃない
これほど完璧なチャンスは二度とないから、今やるっきゃないでしょう。 A chance this perfect will not come a second time, **so I have no choice but to do it **now.