Why I can't say them interchangeably in sentence 2?
1. ... and Jack was standing there, stunned and stupid, trying to understand how a thing like this could have happened.
(Context: Jack lost his temper when he saw his son ruined part of his manuscript and in anger he broke his son's arm. But after that, he regretted doing so)
2. How could I have made a son like that?
(Context: The speaker is annoyed with his son)

Hi, I don't understand why we can say "could have happened" and "could happen" interchangeably in sentence 1 but we can't say "could I have made" and "could I make" interchangeably in sentence 2?

Thank you.

Question

Why I can't say them interchangeably in sentence 2?
1. ... and Jack was standing there, stunned and stupid, trying to understand how a thing like this could have happened.
(Context: Jack lost his temper when he saw his son ruined part of his manuscript and in anger he broke his son's arm. But after that, he regretted doing so)
2. How could I have made a son like that?
(Context: The speaker is annoyed with his son)

Hi, I don't understand why we can say "could have happened" and "could happen" interchangeably in sentence 1 but we can't say "could I have made" and "could I make" interchangeably in sentence 2?

Thank you.

Answer

If you Google "make a son" with the quotes around the phrase you will see that it is not a common phrase. It is not used in common, everyday speech. The Google search only has two pages of results. To your comment, if an author is using the phrase in his book (Game of Thrones), it is because he thinks it sounds okay in that context (a fantasy world).

Answer

You *can* do what you are saying; it is grammatically correct. But it is just not natural speech to use 'make' in that context.

You can make food, or art, because it is in your control. But making a child is not something you do yourself, it is done by sperm and egg (by sex) and by DNA. So you can 'make love' but you cannot make a child! You can conceive a child and you can give birth to a child ('have' a child) and you can raise a child.

Answer

Whew, I had to look up some grammar! Okay, there is a slight difference.

"Could" in "could happen" is not a past tense verb, it is a present tense conditional. The event happened now or could be still happening. See http://english-the-easy-way.com/English_ESL/Could_English_Verb.htm

"Could have happened" is in the past; it is past perfect tense. See http://grammar.ccc.commnet.edu/grammar/tenses/past_perfect.htm#perfect

So, basically, in your example, "could happen" implies a little bit more immediate shock that something happened right now. And "could have happened" would mean a little more time has passed between the event and the present moment.