止まら

In a sentence

• 電車が止まらなかったので乗れませんでした。

  The train did not stop, so I could not board.

  Here, '止まら' is part of '止まらなかった', which means 'did not stop'. It is derived from the verb '止まる', which means 'to stop', in the negative past tense form.

• 私は突然立ち止まらざるを得なかった。

  I suddenly had to stop standing still.

  In this sentence, '立ち止まら' is used as part of '立ち止まらざるを得なかった'. It comes from '立ち止まる', which means 'to stop standing still'. The phrase expresses the necessity to stop.

• この機械は自動的に止まります。

  This machine stops automatically.

  Here, '止まら' is part of '止まります', which is the polite present tense form of '止まる'. It indicates the machine ceases operation in automatic manner.