"would have v3" vs "must have v3" (guess or speculation about a possibility in the past)

Question

I watched a program on ITV channel (British TV channel), in which people call in and ask a psychologist questions about their family problems such as "why is my son doing this", "why is my daughter behaving like this, etc."

While listening one case, one grammar issue caught my attention. It is the use of "would have v3" instead of "must have v3" for a strong guess about the past.

Here is how the native speaker lady used it: One mother is calling the show and complaining the the psychologist that her son does not want to talk to them, avoids them, does not want to visit them even when she was at a hospital after she (the mother) said she did not like his girlfriend a few months ago. So, the mother asks why has her son been behaving like this since then.

And, after she stops, the psychologist lady is making some guesses about what might have happened and why has he been behaving so weird.

In her answer, the lady refers to mother's not liking his girlfriend and says: - "He would have taken that as a rebuke".

I find it interesting, because at school, we were taught that we should use "must have v3" for making guesses or speculating about a past case. However, I see "would have v3" is often used for making guesses about things in the past. Also in this example, the British lady on the TV does not use it and instead she uses "would have v3".

So, is it all right to use it this way? In other words, can we use "would have v3" for strong guesses about the cases that happened in the past?

Thank you.

Note: The link of the video(See 03:08): https://www.youtube.com/watch?v=kEgikcBGE1E

Answer 1

"Would have" can be used as well. It is simply a weaker version of "must have." Where "would have" is suggesting that maybe perhaps this is the case, the other is suggesting that it is even probable.

Scenarios where I'd use must have:

• The cake is gone! Oh, John must have taken it.
• Why are all the bicycles wet? It must have rained.

Scenarios where I'd use would have:

• If you did well on your test, then you would have studied well for it.
• Jake would have taken the bus to get to school if it rained.

Note that the primary difference is that there is more room for error when using "would have". There isn't a certainty, but it is more of a speculation. Whereas when using "must have", it's almost a foregone conclusion.