～しかない・～ほかない

About

しかない can follow any casual (dictionary) form verb to express "no choice but to (verb)".

E.g. こうなったらマイクに頼たのむしかない！ "In this situation, we have no choice but to ask for Mike's help!"

E.g. タクシーが全然ぜんぜん来こないから、もう歩あるくしかない。 "There aren't any taxis (coming) at all, so at this point we have no choice but to walk."

ない can be conjugated to fit the tense/politeness of the sentence.

E.g. 明日あしたどうしても休やすめないから、約束やくそくをキャンセルするしかなかった。 "I simply cannot take tomorrow off, so I had to cancel our plans."

しかない can be replaced by ほかない, which sounds more stiff, and is more commonly used in written contexts.

E.g. こんなに暑あつい日ひが続つづくなら、南極なんきょくに住すむほかない。 "If this heat continues, (we'll) have no choice but to live in Antarctica."

Summary

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  しかない can follow any casual (dictionary) form verb to express "no choice but to (verb)"

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  ない can be conjugated to fit the tense/politeness of the sentence

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  In written contexts, しかない is often replaced by ほかない