In Cambridge Advanced Grammar in Use we read:

  1. We use should have + past participle to talk about an expectation that something happened , has happened or will happen.[page 40, Unit 20]

  2. We use Must have + PP to draw a conclusion about something happened in the past [page 36, Unit 18].

I dont understand the difference between the "drawing a conclusion", and "predicting/forecasting/guessing an occurrence"

Consider :

John isn't home yet. He -------------------have got in trouble.

Is there any difference if we use Should instead of Must and vice versa ?

2 Answers 2


There is a huge difference between the two sentences:

He must have got into trouble.

means that something has occurred which may or may not have been John's fault

He should have been home by now.

means he is not at home, but enough time has passed that it would have been achievable

He should have got into trouble.

means he did not get into trouble but he would have been better off if he did (it is an awkward sentence in that respect)

Drawing a conclusion involves determining an outcome using facts. It is a form of prediction and forecasting. Guessing usually means trying to determine an outcome without any facts ie a wild guess

  • I would say "drawing a conclusion" is an act of "forecasting" not the other way around as you put it. I would't say they are completely interchangeable. Forecasting is predicting the future, which can use qualitative (logical) or quantitative(numeric) techniques. "Drawing a conclusion", to me, has more of a qualitative aspect."He must have got into trouble" means "something bad has happened". "He should have brought the milk home" means he did not get any milk.
    – Peter
    Commented Dec 5, 2015 at 0:46
  • I see your points. but, given to the (2) in the question, we can use should to talk about something has happened. This is confusing.
    – Cardinal
    Commented Dec 5, 2015 at 0:54
  • I know we can say:"We should have wait for him" (but we don't) , somehow a negative state, but this is not the case related to (2). This usage must have been very uncommon in English, (2) I mean
    – Cardinal
    Commented Dec 5, 2015 at 0:57
  • "should" is something that is supposed to happen, but has not. For example: "if I go out in the rain, I should get wet", you expect to get wet. The other use of "should" is obligation: "he should take out the garbage", but maybe he hasn't yet
    – Peter
    Commented Dec 5, 2015 at 0:58
  • That form expresses regret over not doing something (obligation) "We should have waited for him", "I should have watered the plants", "I should have known..."
    – Peter
    Commented Dec 5, 2015 at 0:59

We'll start from the scratch.

When we are certain about something, we use must:
You must be tired after working the whole day.

To talk about certainty in the past, we use must have:
I lost my keys. I must've left them in the car.

(Note that this application of the modal must doesn't imply to be an obligation.)

the right thing to do
When want to provide an advice or say that something is the right thing to do, we use should:
You should eat healthy food.

When there was a way to do the right thing and you did something else, we use should have to show the correct way to behave was:
You should have eaten healthy food.

  • Thanks for the answer. My question revolved around the definition of the "drawing conclusion". I thought it is possible to draw wrong conclusion, therefore, let suppose I draw a wrong conclusion saying " he must have got in trouble", this does not mean "he has got in trouble", since I was wrong.
    – Cardinal
    Commented Dec 5, 2015 at 20:00

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