have sth on the tip of one's tongue
1.I have her name on the tip of my tongue.
她的名字就在我嘴边,却说不出来.
2.I have it on the tip of my tongue to ask him why he left home,but the uselessness of it is apparent.
我差点问他为什么离家,但显然问也无用.
或者直接说"I can't find the word I'm looking for"