忍不住【rěn bu zhù】

忍不住 + verb says the urge was too strong and you did it anyway – laughing, crying, eating, asking. It is built from 忍 "hold back" and 不住, which says the holding didn't last.

The verb usually ends in 了 or carries 起来, because it reports something that did happen.

On its own

With no verb after it, 忍不住了 means "I can't take it any more" – the urge has won, and what it is comes from context.

忍不住 or 受不了

忍不住 is about an urge inside you. 受不了 is about something outside you that you can't put up with.

• × 我忍不住这么热的天 → 我受不了这么热的天

Not 不能

English "can't help" tempts a 不能 construction. It doesn't work: 不能 says something isn't allowed or possible.

• × 我不能忍住笑 → 我忍不住笑了

The written 不禁 has the same meaning and has its own point.