past tense - Present Perfect with the word "ago"?

First, I check that my native language is not English, and the problem described below met on one of the exams.

With this sentence:

"The most important news is that my parents [...] a new restaurant a few

weeks ago."

Is the correct tense to use the Present Perfect (have opened) or Past Simple (opened)?

I know that the word "ago" but rather that time Past Simple, but is it also possible to use the present perfect in some cases?

I would be grateful for clarification.

Answer

The present perfect tense, as you asked about, is when Event 1 (your parents opening a restaurant) happened at an unspecified time before Event 2 (your present conversation).

The reason that there is obfuscation and confusion is because of "a few weeks ago".

If you had just stated "The most important news is that my parents have opened a restaurant.", it would be clear that the perfect tense is appropriate because Event 1 was completed at an undisclosed time before Event 2.

"a few weeks ago" adds definiteness, but not exactness.
For example, if you stated "My parents opened a restaurant two weeks ago.", it is clear that you could not use the perfect tense with a definite time.

That being said, it is my opinion that you giving a time frame for when the restaurant opened has removed the indefiniteness of time, and you should not be using the perfect tense.

Have a nice day.