Magnificent Modals Part 3: Talking about certainty and possibility…


Modal verbs…again!

In previous posts, we’ve looked at  general modal rules and modals of obligation in Part 1; modals of ability and permission in Part 2; and we’ve decided to let you know about modals of certainty and possibility (modals of deduction) here in Part 3. Let’s get flying and hopefully you’ll find it useful…!


Where do you think this picture was taken?



What can you guess about the area this picture was taken in? Do you think it rains much? Is it likely to be hot?

We can use modals of deduction to give our opinions about how likely we think the answers to these kinds of questions are. In this blog, we’re going to continue looking at modals and their use in English.

Many students make mistakes with the correct use of these modals, so here are a few reminders and explanations to help you.  (In a previous blog, we looked at the basic grammar rules of modals – if you feel you would like to look again, click here).

First, let’s look at when we are sure about what we think:

must and can’t

We use must when we believe something is certain. So, looking at the picture above, we can see that it looks like a desert. This type of area is usually hot in our experience,  and so we can say “It must be hot during the day there“.

The opposite, when we talk about things we are certain are NOT true, is can’t. So, as there are very few plants and looks very dry in the picture, we can say “It can’t rain a lot there“.

We use these modals only when we are certain in our opinion or knowledge of the world. If we know as a fact that something is true, we don’t use must or can’t. I can say “This photo has a lot of rocks in it” (because I can see a lot of rocks and this is a fact, I use the present simple) but not “This photo must have a lot of rocks in it(because I can see a lot of rocks, so I am not using my knowledge of the world to deduce that they are there).  For a reminder of how we use the present simple, click here.


  • Remember the opposite of can’t is NOT can: for when we think something is true we use must (see above) or could (see below). “It can rain very little there” is NOT correct.
  • We can use can for general  possibilities (i.e. we know they sometimes happen)  but not for a possibility in a specific situation (like the Irish dancing girls above). So, “The girls can be in a dancing competition is NOT correct as this is a specific occasion, but “Dancing can be difficult is correct because this is a general possibility – every time we dance, there is a possibility that we could find it difficult.
  • Also, note that in American English you can say It must not rain a lot there but this is NOT correct in British English or Irish English.

Now look at this picture:



Can you say what happened?

This shows something that happened in the past (the car crashed before now). To say what we think happened in the past, we use must have + past participle or can’t have + past participle, e.g. “The driver must have lost control” or “The driver can’t have been looking where s/he was going”.

We can also use couldn’t instead of can’t here: “The driver couldn’t have been looking where s/he was going”.

Talking about possibility and probability

may, might and could

We can also use the modals may or might if we are not certain of what we think happened but are saying what we think is likely. I don’t know where the driver is now but if I think it is likely that s/he is in hospital, I can say, “The driver might / may be in hospital”.

To talk about possibility, (if I think it’s possible the driver is in hospital), we use could, e.g. “The driver could be in hospital”. 

  • May, might and could can also be used in the present or in the past. They follow the same rules to go in the past: use have and past participle. “The driver may / might / could have had a drink”. 
  • We can use well after may, might  and could to show that we think the possibility / probability is quite strong, e.g.The car may well have to be scrapped” = I think it is very likely that the car will have to be scrapped.
  • The negative is may not / might not. This means that there is a possibility that something didn’t happen, e.g. “The driver might not have seen the tree”. We can also use couldn’t, e.g.  “The driver couldn’t have seen the tree” . However, this means that there was NO possibility that it happened (the same as can’t – see above).

Couldn’t is only correct when deducing about the past, but it is not correct in the present. “The driver couldn’t have seen the tree” is correct; but “The driver couldn’t be an experienced driver is not.  



Talking about what we assume to be true

Should and shouldn’t are used when we make an assumption about what is probably true, if everything is as we expect. We can say  “Annie should be home by now” if we expect Annie to be home as it is after her normal time for coming home.  The negative is shouldn’t, e.g. “It shouldn’t take long to drive back”. 

We don’t usually use should for negative events. We use will instead, e.g. “The underground will be very busy now” The underground should be very busy now.



 Well, that’s enough theory! Now it’s time for some practice:

youtube-logo2For a video giving an explanation of these modal verbs, and also some revision of all modal verb rules, go to this YouTube video.



Choose one of the conspiracy theories below and change the sentence using modals of certainty and  possibility above to make it have the amount of likelihood that you think is really so. Do your fellow-students agree with you? Can you think of any other conspiracy theories (real or from a book/movie) you can use this language about?

  • President Kennedy (=JFK) of America was assassinated by the CIA helped by the Mafia.
  • An alien was dissected at the Roswell American air force base.
  • Jack the Ripper was never caught because he was someone important and the police protected him.
  • The Apollo moon landing was a hoax filmed in a studio by Stanley Kubrick
  • King Arthur was a real person and was a Roman fighting with Ancient Britons against the invading Germanic Anglo Saxons

(This activity was adapted from Click on the logo below to find this and other exercises.




There are some online exercises to practise using these modals here and also here.  


Be Sherlock HolmesCan you work out what happened to D.B. Cooper? In this site you can watch a short film about a famous criminal, do some listening comprehension exercises and practise some crime vocabulary; as well as practise modal verbs for past deduction (may / might/ could / can’t / must have + participle) Click here to go to the site.


Why don’t you practise your writing skills and write a paragraph about one of the images on this page – you could exchange writing texts with a friend and do some peer-editing to develop your writing .

Have fun practising, and don’t forget to let us know how you got on by leaving a comment!


Birds flying image from

Namibian desert image from:

Irish dancing image from

Car crash image from:



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