How is, "Many young people were disappeared at that time," grammatically incorrect?

You can say "were disappearing" or "had disappeared". Your usage of "disappeared" in the sentence above is a slang term, and, therefore, would not be considered correct grammar.

As Helmuth said, the new usage is a transitive form of the verb, so the structure you used is grammatically correct. The new usage allows you to say "The regime disappeared many people who disagreed with them." Your statement is a passive form of the same usage.

It does not appear in my Chambers Dictionary from 1983, or my Oxford Dictionary from 1992.

What an interesting question! The only definition in my dictionary is for "disappear" as an intransitive verb. However, arising from the atrocities in Argentina, "disappear" is now listed in online dictionaries as a transitive verb, the form in which it's used in your sentence. (So your sentence is grammatically correct.)

Disappear (trans.verb) means to cause to disappear; to kidnap and execute (persons) in a clandestine program of political terror.

This is wrong─and would never be right─because to use the word 'were' before the word 'disappeared' would make this sentence's meaning faulty. Disappeared is another word for vanished, and you cannot write something like 'young people were vanished at that time' either. Why? People can't disappear or vanish any person, place, or thing. (It's a coherency error.)

This word is also transitive, which means that it must be APPLIED to a noun.

Many young people disappeared at that time.

is correct

Transitive :(

I think it sounds stupid...

http://www.merriam-webster.com/dictionary/disappea...

its not incorrect