Grammar JLPT Level N3 - Lesson 15

しかない is a commonly used expression in Japanese that is partnered with verbs. It is a combination of しか, the adverbial particle, and the い-Adjective ない. This structure may be translated as 'to have no choice but (A)', or 'there is only (A)'. しかない regularly emphasizes the point that nothing apart from (A) is possible.
To use しかない, simply attach it directly to the non-past form of whichever verb you would like to highlight as being the 'only' possible action.

• こうなったら警察(けいさつ)を呼(よ)ぶしかない。
  If it's come to this, there is no choice but to call the police.
• もう電車(でんしゃ)が来(こ)ないから、車(くるま)で行(い)くしかない。
  Since trains won't be coming anymore, there is no choice but to go by car.

Unlike だけ, which implies that one particular option was chosen (out of several), しか always insinuates that (A) is the only option, and therefore will sound much stronger than だけ. Also, as only one option exists, it will often be assumed by the listener that some particular circumstances resulted in (A) being all that can be done.

• この時間帯(じかんたい)はピザハット以外(いがい)どこも開(あ)いていないね。こうなったらピザを頼(たの)むしかないね。
  At this hour, nowhere is open other than Pizza Hut. Now that it's come to it, there is no choice but to order some pizza, right?
• 今月(こんげつ)は休(やす)めないから、コンサートに行(い)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.