Change into active voice

The dead saw him.

but the dead can't see

The dead can't see in your original sentence either!

I didn't say that the dead can see .

you only misunderstood me.

What did I misunderstand? "He was seen by the dead" is a grammatically correct sentence, although it does not represent a true fact because the dead cannot actually see. Exactly the same is true of "The dead saw him."

the answer is

they saw him by the dead

mohzayatthe answer is

they saw him by the dead

No. You need to review active and passive.

CJ

PS. However, these make an active-passive pair.

He was seen by the Dead Sea.
Someone saw him by the Dead Sea.

CJ

mohzayatthe answer is

they saw him by the dead


This absolutely makes horse sense....[:^)]

So, you intended "he was seen by the dead" to mean "he was seen near the dead"???!! Okay, I must say that I feel we've been taken advantage of. Maybe I've misunderstood again, but it seems as though you have deliberately posted an ambiguous sentence, and then, after three people attempted to answer what we all took to be the obvious meaning of the sentence, you answer your own question b

I agree completely with khoff.

It would have been far more appropriate if you had simply asked whether the sentence you posted is likely to be understood with the meaning you intended it to have, and/or whether a native speaker of English would be likely to use the sentence at all.