"Đây là một kết luận sai."

Translation: This is a wrong conclusion.

[JamesTWils]

I do not know why, but "a wrong conclusion" sounds odd to me. "The wrong conclusion" or "an incorrect solution" both sound perfectly fine.

[Johnathan363023]

You are correct. In English grammar follows the principal of (A/An -then- The) or Plurals.

This shows that the first time a non-plural subject is brought up we must use a/an with the noun. Every time after that we must use the word "the" for the noun.

This is a chair. The chair is red.
(If we say "This is the chair" then it implies that we previously talked about the chair and this is the chair that I told you about. Alternatively, if we say "A chair is red" then it is unrelated to anything we were talking about previously.)

So when we say "this is the wrong conclusion" it implies that there may be many other wrong conclusions that we are talking about and this is just one of the many wrong conclusions.
When we say "this is the wrong conclusion" then it implies that this is the only wrong conclusion.

[LostinVN]

I agree with James T. Wilson. A sounds strange.

[LostinVN]

I think it's because we already know the conclusion you are referring to, so it changes from 'a' (one of many) to that one we are talking about therefore 'the' is justified.

[ingochris]

"This is a false conclusion" works as well.

[AnCatDubh]

It’s not! It clearly shows here that the decrease of pirates causes global warming!

[iblinguist]

This conclusion is wrong. Duolingo seems to use "một" much more often that is used in real life.

[OngDuDu]

This is a wrong conclusion. Why not " This conclusion is wrong" as an alternative answer- that is more natural for English native speakers.

[Steve804244]

I went with the literal translation (This is a wrong conclusion.) because DL tends to like literal translations, even when they result in odd sounding English usage (a wrong conclusion). The most common way to express this in English is: This conclusion is wrong.