Tā zhāngkǒu xiǎng shuōhuà, què bùzhī cóng hé shuōqǐ.
He opened his mouth to speak but didn't know where to begin.
Tips
usage
张口 literally means 'to open the mouth' but idiomatically means to start speaking, especially when making a request. 张口就来 (zhāngkǒu jiù lái) describes someone who speaks with ease or rattles things off without thinking.
