could...be

sft MDo both of them sound like questions?

The first one is incorrect.

sft MHow could be he so foolish?

This doesn't sound natural.

sft MHow could he be so foolish?

This does sound natural.

sft MAre both of them sounds like an interrogative question?

Only the second sentence is acceptable.

Thank you!

sft MAre Do both of them sound like an interrogative questions?

What about this?-

What could be the value of this variable?

In that sentence I am using 'be' just after could. And I found this sentence on a English book.

sft MWhat could be the value of this variable?

That is acceptable, as is "What could the value of this variable be?

How could he be so foolish? That is acceptable but "How could he be so variable?" is not.

sft MI found this sentence on in an English book.

fivejedjonHow could he be so foolish? That is acceptable but "How could he be so variable? is not.

I must have got lost in this thread. The unacceptable version is unacceptable because of the choice of the word "variable"? Or because of the word order? If it's the latter case, I'm not seeing why.

CJ

Sorry for the confusion. It appeared to make sense when I wrote it, but I seem to have run thoughts together without noticing it. Try again.

What could be the value of this variable? is acceptable, as is What could the value of this variable be? I prefer the second.

How could he be so foolish? is fine, but How

Oh. OK. For a moment I thought I was going crazy. (It happens.) All is clear now.

CJ