So I know that 何で can be used to ask 'why' or to ask "with what" in a
negatively structured sentence. Like, お姉ちゃんは何で学校に行きますか.

But when I wanted to learn how to say, "idk what to say" a Japanese person
suggested the same usage to me, like 何で言えばいいか分からない so i'm curious now why
isn't it 何を?

I'm a newbie learner sorry if this looks funny.
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grammar
How does 何で usage exactly work with 言う?
It was **何て** 言うの. 何て when used in this context (question form) simply means
"what".

And I'll add this, when you say **何を** 言えばいいか分からない it feels like you are
describing somebody else who's at a loss for words, "He doesn't know what to
say", not "Idk what to say". I don't have a dictionary backing me up, but it
just feels this way after reading a lot.
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> How does 何で usage exactly work with 言う?

Why not? But first you have to see that 何で can mean two different things:
`why` (in which case it's read as 何{なん}で) or `by what` (in which case it's
read as 何{なに}で)

So for your two example sentences, they're both correct, but it's just that in
the first, the 何で means `by what`

> Q: お姉ちゃんは何{なに}で学校に行きますか  
>  A: 自転車で学校に行きます

> Q: By what means does the sister go to school?  
>  A: By bike

In the second sentence, the 何で means `why`, and is analogous to なぜ or どうして

> 何{なん}で言えばいいか分からない  
>  I don't understand why is it good if I can say it

In short, 何{なに}で and 何{なん}で are different words that mean different thing.
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As other answers pointed out, it was

> 何て言えばいいか分からない

何て means "indefinite way" or "how" and it's the adverb form of 何.

It literally means: "If I say (it) in an indefinite way, I wouldn't understand
if it's good". -> "I don't know how to say it"
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