Verbs and Modals
Modals of obligation and probability
Say how strong an obligation is with must, have to, should and ought to, and say how sure you are with must, might and cannot.
Modal verbs do two quite different jobs, and this is the year the two split apart. One job is obligation: how much choice you have. The other is probability: how sure the speaker is. The same little word can do either, and only the sentence around it tells you which.
Obligation, strongest first
must and have to leave no choice at all. should and ought to give advice — the listener may still decide otherwise. Between them there is a whole world of difference: You must apologise is an order; You should apologise is a suggestion.
The trap that catches everybody
must not and do not have to are opposites, not neighbours. You must not go means it is forbidden. You do not have to go means nobody minds either way. Getting these the wrong way round can turn a warning into permission.
Must has no past
There is no past form of must. For yesterday you reach for had to instead: I must leave now becomes I had to leave early yesterday.
Now the other job. When you are working something out from the evidence in front of you, English gives you a three-step ladder. At the top, must — you are as good as certain it is so. In the middle, might or could — it is one possibility among several. At the bottom, cannot — you are as good as certain it is not so.
Worked example
The kitchen light is on and there are wet footprints by the door. What can you say about your brother?
Start with the evidence, not the guess: a light on, wet prints leading in.
Probability modals are always an argument from something you can actually see.
Try it together
Let us work through one situation together, choosing the modal the evidence earns.
The situation: the school gate is padlocked, the ground is empty, and the notice board says the sports day was moved to next week.
1.Somebody asks whether the sports day is happening today. The evidence rules it out completely. Fill the gap: "It ____ be today."
Have a go
Your neighbour's car is in its usual place, but every window in the house is dark. Which sentence fits the evidence best?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.