他突然想起了忘记关煤气。

He ___suddenly remembered that he forgot to turn off the gas.

他突然从座位上站起来，吓了我一跳。

He ___suddenly stood up from his seat, which startled me.

他突然停下脚步，好像看到了什么可怕的东西。

He stopped ___suddenly, as if he saw something terrifying.