The difference between "must," and "have to," can sometimes be confusing. There aren't any hard and fast rules, but in this post we going to explain to you how these rules are usually used by native speakers. We use must / have to + infinitive to say that it is necessary to do something. Note that there is no, "to" when we use must.
So let's look at must,first.
The first thing to remember is that must is only ever used in the present tense. For all other tenses, in order to talk about something being necessary, we use have to.
The second thing to remember is that we usually use must when the sense of obligation for doing the thing we feel is necessary comes from us, not from somebody else.
I must get my hair cut soon, it's getting really long. (In my opinion.)
We must remember to phone Sophie - it's her birthday today. (I don't want to let her down.)
Now let's take a look at have to.
We usually use have to for rules, or for things that other people think are necessary for us to do.
I have to leave now, otherwise I'll be late for work. (My boss thinks it's necessary for me to be on time... What a jerk!?)
I can't come to the party until later, I have to pick up my boyfriend from the airport. (He's asked me to.)
So, to sum up: we can use either must, or have to in the present tense, to create a sense of obligation, but the words we choose show where we think the obligation is coming from. If we need to show obligation in any other tense, we use have to.
Practice Time
Let's have a look at some pairs of examples. See if you can pick which sentence is correct (or if both are OK.)
(a) I must to finish my homework before I can go out. (b) I have to finish my homework before I can go out.
(c) I couldn't come to the party because I had to work last weekend. (d) I couldn't come to the party because I must work last weekend.
(e) I'll have to finish vacuuming before I can come. (f) I must to finish vacuuming before I can come.
(g) I must get to work on time today. (h) I have to get to work on time today.
(i) We have to buy our train tickets before we get on the train. (j) We must buy our train tickets before we get on the train.
Answers...
(b) is correct, because we don't use "to" after must
(c) is correct, because we can't use must for the past tense
(e) is correct, because we can't use must for the future tense (though we could use must with the present simple to indicate the future - I must finish vacuuming before I come out.)
both (g) and (h) are correct, but they have slightly different meanings:
– (g) gives us the idea that you really want to get to work on time today for personal reasons; perhaps you want to make some phone calls before you start work, or perhaps you want to get into the boss' good books
– (h) gives us the idea that if you do not get to work on time today, there may be some kind of negative consequence such as being fired or missing a meeting
both (i) and (j) are correct, but they have slightly different meanings
– (i) tells us that the rules are that train tickets are to be bought at the station, not on the train, and you cannot get on a train without a ticket
- (j) tells us that for some reason the speaker wants to buy his train tickets before he gets on the train, or is perhaps reminding himself out loud to buy the tickets
So! That's pretty much it! Next time you hear native speakers using these expressions, hopefully you'll understand them a bit more clearly. Wondering about the negatives, 'mustn't,' and 'don't have to?' We'll talk about those in our next grammar post... see you then!