1 - you need a comparative. "Not such a good player" stands alone. 2 - it has to do with the indefinite "a". "The Opera House" is assumed to be an obvious thing, so it makes little sense to then explain which one you mean... ie. if there is only one. It's like saying "the sun, which is in the sky". 3 - "the responsible people" implies there are irresponsible people. "People concerned" is a condensation of "(the) people (who were) concerned (with this event)"

This test is advanced and is not just about grammar. #1. This one is grammatical. It is clearly comparative, so use "as....as..." #2. "The Opera House" is the NAME of the actual opera house and refers to one building only. So, if you met him at The Opera House, it is obvious which one. You don't need to specify it as the one you went to. It has nothing to do with the definite or indefinite article. This one is an advanced test question! #3. "responsible people" is just nonsense, I'm afraid. This one tests your knowledge of English. I'm afraid you didn't do well with this one. I'm afraid I have to agree somewhat with Tulpan here. Your comprehension needs improving a lot.

excuse me ı dont undrstand

может вы поможете меня учить русского языка

Peachy's answers (above) are spot on as usual... ))