他一开口就滔滔不绝。

When he started to speak, the words just spilled out.

《牛津词典》

我知道我一开口就会哭。

I knew I'd start crying if I opened my mouth.

youdao

图图不常开口，但他一开口，就会变得十分愚蠢。

Once Tootles began, which was not very often, he had a silly way of going on.

youdao

because if you go out in public and you start talking to someone, you'll just burst into tears.

因为在外面的时候，你一开口跟人说话，眼泪就禁不住留下来。

I feel like 课堂 - SpeakingMax英语口语达人