I think it may be "mute as a goldfish"? Or you could possibly say, "He is my silent partner." That would mean that you are very close friends and do everything together, but he doesn't speak. Um... that might not be the proper use of the term "silent partner", but I seem to recall it being a wink-nudge sort of thing when I was younger.
How can I say "He doesn't speak at all" using an idiom?Help me please!I know that may looks strange and easy question for you,but I'm not English and I really need your help!Thenk you!
There are several possibilities, but, of course, each has its own shade(s) of meaning:
Tight-lipped refers to someone who talks very little. It can refer to someone who is keeping a secret.
Close-mouthed means the same thing.
Taciturn is not an idiom, but t means almost exactly what you are looking for. I t describes someone who is habitually unwilling to speak.
Laconic also not an idiom, but expresses your idea, too.
As mute as a fish is not very common, but it would be instantly understood.
S/He is as silent as the grave. To me this means s/he can keep a secret, but also may just mean s/he doesn't talk much.
Cat got your tongue? This is asked of a person who is silent when asked a question, or is otherwise expected to speak.
Quiet as a cat refers to someone who keeps quiet in an attempt not be noticed.
Quiet as a mouse refers to someone who makes no noise at all. It is not a direct reference to not speaking, but certainly includes it.
I'm sure there are more. Perhaps other posters will add to the list.