Must for Speculation and Logical Certainty. Modals 205
The must be feeling happy. They must have arrived
We use must to speculate about the present and the future and we use must have to speculate about the past. When we are using must in this way we are talking about something that we are almost certain something is true. Logically the situation must be like that.
For example: They left two hours ago and the journey only takes an hour and a half so they must be there by now. (speculation about the present)
You have been traveling all day, you must be tired.
You look like you have a terrible hangover, you must have drunk a lot of brandy last night. (speculation about the past)
Speculation of certainty about the past is the only use of must have plus past participle. We do not use this construction to talk about past obligation. For past obligation we use had to. When using must to talk about the present or the future, it is necessary to look at the context to decide if the meaning is speculation of certainty or obligation.
For example: He must be working at the moment. (speculation)
He must be at his desk working before eight o'clock in the morning. (obligation)
Notice that the opposite of must for speculation is can't or couldn't in the present. I the past the opposite of must have plus past participle is can't have plus past participle or couldn't have plus past participle.
For example: He looks sad, he can't be thinking about me.
It's four o'clock in the morning so he couldn't be working.
She looks tired, she can't have slept at all last night.
He couldn't have committed the crime because he was with me when it happened.
The following video explains Must for Speculation and Logical Certainty:
Hi there. Must for Speculation and Logical Certainty. OK. In the present we very often use this in the continuous tense. Where's your brother? Oh, it's eleven o'clock, he must be working. He must be working. It is a logical certainty that at this moment he is working. I am certain that he is working. He must be working. Where is your brother. or where was your brother yesterday? I don't know, I didn't seen him all day, he must have been working. Speculation or logical certainty about the past. Notice this is a case of must in the past but only for logical certainty. Erm... I arrive home, my brother is not there, he must have gone to work. The logical certain with the evidence of the situation is that he has gone to work. My friends are in America, they must be having a good time. I am certain, it is a logical certainty that at this moment they are having a good time, they are enjoying themselves. They must be enjoying themselves. OK. They must enjoy, that's an obligation, they must be enjoying, a speculation, a certainty. So, thanks for watching and I'll see you in another modal video. I'll see you soon. Bye