what's the meaning of "心の中だけで笑わない"?

Question

I have seen this sentence in a japanese tv show. I think it should mean:“only in your heart, you don't laugh.”
But the right explanation is,

“Don't just laugh in your heart ”

(by the way it tells you that "Do not just feel laughter in your heart, but express it outwardly.)
I still don't understand even search many websites.
there is the same(maybe?) sentence:

口だけで行動しない

it means:“only talk and no action”
why there has the difference between this two sentences?
And what's the meaning of the above sentence?

心の中だけで笑わない

I would be appreciate it if someone can tell me.:)

Answer 1

The で in the first sentence is a particle denoting a place where some action happens and, judging from the translation, the sentence was said as a negative imperative sentence. (It would sound weird as a statement.)

It can be understood as:

心の中だけで笑わない（＝笑うな）。
Don’t laugh only in your heart.

This implies you should laugh in other places as well.

The で in the second, on the other hand, is a copula as 口だけ stands as a predicate by itself.

（彼は）口だけだ。
He is all talk (lit. He is only the mouth).

This is combined with another independent sentence（彼は）行動しない, which just happens to be a negative statement.

（彼は）口だけで（あって）、行動しない。
He is all talk and doesn’t act.