1. I didn't know you had such great talent in ballet.

2. I didn't know you had such a great talent in ballet.

(Which one is correct? Do we have to insert 'a' or not?)

3. Do you really mean it, Dad?
(#3 is the answer to the previous statement. What does "mean it" mean?
4. Do you really mean that you didn't know I had such great talent?
5. Do you really mean that I have such great talent?

Does #3 mean #4 or #5?)

2. is better.

3. The child is questioning whether the father is serious. Does his dad really mean that he had a great talent?

Yes. 4. and 5 mean the same as 3.

1. Both sentences are grammatically correct, but they have slightly different meanings.

In the first sentence, "I didn't know you had such great talent in ballet," the speaker is expressing surprise about the person's talent in ballet. It implies that the speaker was previously unaware of this talent.

In the second sentence, "I didn't know you had such a great talent in ballet," the speaker is also expressing surprise about the person's talent in ballet. However, by including the article "a," it emphasizes that the person has a specific, single talent in ballet.

So, which sentence to use depends on the intended meaning and emphasis.

2. "Mean it" means to sincerely or genuinely intend or believe something. When someone asks "Do you really mean it?", they are expressing doubt or seeking confirmation about the sincerity of a statement or promise made by another person.

In this context, "Do you really mean it, Dad?" is asking for confirmation about the father's previous statement. The child is seeking reassurance that the father truly means what he said.

3. Statement #3 does not necessarily mean either #4 or #5. It is an open-ended question without enough context to determine the specific intent. The child is simply seeking clarification or confirmation from the father regarding his previous statement. The response could be either confirming that the father means #4 or denying that the father means #5, depending on what the father's original statement was.