I must be going.

[tom3m]

I have a question about this sentence: I must be going. I do not understand why there is the verb to be + present continuous form. I have thought that only infinitive can be used after must. Can you explain the difference if any.

[5jj]

It is an infinitive form, a continuous one.

[dilodi83]

How can we know how and when to use "I must be going" and "I must go"?
Do they mean the same or are they used in different contexts?

[5jj]

How can we know how and when to use "I must be going" and "I must go"?
Do they mean the same or are they used in different contexts?

It depends on the context. In this particular example, there is little practical difference. In the following pair there is a difference between the meanings of the continuous and the non-continuous form:

You must be doing your homework when I get home - You must start before I get home.
You must do your homework when I get home.- You must start as, or immediately after, I get home.

[dilodi83]

It depends on the context. In this particular example, there is little practical difference. In the following pair there is a difference between the meanings of the continuous and the non-continuous form:

You must be doing your homework when I get home - You must start before I get home.
You must do your homework when I get home.- You must start as, or immediately after, I get home.

So, in the first one the progressive form means that I have to go on doing something that I have already begun to do before; and in the second one, if I do not use the progressive, it means that I have not begun to do something before, but I'm starting doing it right now...Am I correct?

[5jj]

So, in the first one the progressive form means that I have to go on doing something that I have already begun to do before; and in the second one, if I do not use the progressive, it means that I have not begun to do something before, but I'm starting doing it right now...Am I correct?

You are correct when we are talking about those two sentences.

[dilodi83]

But what about the general rule?

what about this kind of sentence? - I think you must treat her an icescream
- I think you must be treating her an icescream.

[5jj]

wWhat about this kind of sentence? - I think you must treat her to an ices cream
- I think you must be treating her to an ices cream.

The first one only is possible when 'must' imposes an obligation.

ff 'must' suggests logical probability. the second is possible, and it can have future reference. For example, a friend sees me walking with my young daughter in the direction of an ice cream stand; my daughter is looking very happy, and my friend says, " I think you must be treating her to an ice cream".