Hey there! This is TOP episode 104: Must and Can’t. Modals

My name’s Ola and I am an English teacher. My goal is to help you start speaking English with confidence and get rid of speaking barriers. It’s time you started speaking English fearlessly! I’ve been there. I was unable to speak English for many reasons that now I call a language blockade. Today I teach people like you how to speak English with confidence. Go to my website for full transcripts and worksheets to each episode. Happy learning! 

Hello and welcome to another episode. It’s grammar today, I hope you’re excited! We’ll discuss only two modal verbs. I think it’s sensible putting them together because they show contrast. I hope you’ll learn a lot today. Let’s do this. One more thing, please stay till the end because you need to practice speaking and that happens at the end of each episode. Secondly, please subscribe, rate, comment, like and share, do whatever you can to support this podcast. Thank you! Let’s jump right in!


Today we’re looking at must from a different perspective, different from obligation. Listen, today we are not looking at ‘must’ as a modal verb of obligation. Today we’re talking about ‘must’ as a verb of probability. Modals of deduction. Because sometimes we act as detectives and estimate the probability of something to happen. There are a few modal verbs of probability: must, may, might, could, can’t, can, but also will and won’t. Ok, but you know me I’d rather you spent those few minutes on one modal verb and get a good grasp of it than learn a whole bunch of modals not remembering any. 

‘Must’ is a modal verb of probability. How probable are events described by this modal? Fairly sure. In other words, when you use ‘must’ it’s because you are quite sure that this is a good guess. You believe this is true. It’s not 100% because just like Sherlock Holmes you are making deductions about the facts. Your deductions are based on some solid evidence. I would say that you’re about 90% sure. 

Where is she? She must be on the bus. She must be on the bus. I believe she’s there, I’m fairly sure about that assumption for the fact that she texted me yesterday, she wrote she would go by bus. I’m quite sure she’s there. 

You look through the window and see people wearing shorts, tank tops, sunglasses, flip flops or sandals. And you think to yourself: It must be hot. Sherlock Holmes, remember. 

You’re looking at your friend, who has been working outside all day in this scorching weather and he’s clearly knackered. And you say: You must be tired!

Ok, now. What about the past? What is the past form of ‘must’? In this case, let me repeat that we’re discussing deduction, modals of possibility: the past form of ‘must’ is ‘must have done’. Must have been. Must have seen. Must have gone. Must plus have (always have, even for the third person singular), she must have. So! Must plus have plus past participle which is simply your main verb in the third form. Must have done, must have sung, must have drunk. 

Picture yourself in front of the window, it’s early morning. You see that the pavements are all wet and it strikes you that: It must have rained at night. It must have rained at night. Really focus, close your eyes, see the situation in your mind’s eye, get familiar with these sentences. You’ll see. When the right time comes those phrases will jump out of nowhere. In a perfect form, in a perfect context. Trust me. 

One more situation to visualise. Close your eyes if it helps to focus. Listen. You are watching a film. It’s not that you remember the plot, because you don’t, still you kind of recognize the dialogs, and the actors or the characters seem familiar. And at some point you realize: I must have seen this film before.

Ok, that’s all I wanted you to focus on when it comes to ‘must’ and now let’s move on to ‘can’t’, shall we?


‘Can’t’, my dear listener, is the opposite of ‘must’. I know this sounds ridiculous. And I know why. It’s because you associate ‘can’t’ with lack of skill or ability, so this is most probably in your understanding a modal verb of ability, which is correct. But that’s not the whole truth. Can’t serve us as a modal verb of probability. So, again, deduction, assumption. 

We’re making assumptions based on what we know and we consider those assumptions fairly accurate. Use it when you are fairly sure something is not possible. With ‘must’ we were quite sure that something was possible. Here we’re quite sure it’s not. And that’s why it is the opposite of ‘must’. You believe that something is not true. 

Visualize. You are on holiday, but unfortunately somebody from the office calls you. You answer the phone, and you’re told that you are promoted. You’re going to get a pay rise as well. Splendid! However, you know this is a mistake of some kind, because you had discussed this with your boss before your holiday. He made it crystal clear that the company cannot offer any pay rise at the moment. Maybe the next quarter. So, you think to yourself: This can’t be true. 

One more picture. You go to a clinic, and your doctor is really young. You connect the dots in your head and have this disturbing feeling: He can’t be a doctor. He’s way too young.

This is so much fun. One more thing to consider. What’s the past tense of ‘can’t’ in this meaning? It’s not ‘couldn’t, please focus’, it ‘can’t have been’. Please don’t hate me, I didn’t make it up, it is what it is. Can’t have been, can’t have done.  

You come back home and tell your partner, I’ve been examined by such a young doctor! He can’t have been a doctor really. He was way too young. He can’t have been a doctor really. He was way too young. 

Visualize one more situation. You are in the kitchen, you need eggs to make a dish. You remember eggs were on your shopping list, still they’re not in the fridge. You’re astonished but you accept the fact that: I can’t have bought them yesterday. See, there’s one more thing I want to add. It’s possible that must have sounds more familiar than You can’t have done. It’s not without a reason. ‘Must have’ is more common than ‘can’t have’. 

They are not synonyms, as you already know but you can change the sentence so that ‘must have’ fits. For instance, instead of saying: I can’t have bought those eggs you may wanna say: I must have forgotten about eggs. 

One last thing, can’t have done can be replaced with could have done. Let’s see how it works:

He can’t have been serious.

He couldn’t have been serious.

It works, and the meaning is the same. 

All good, but we need to put this theory into even more practice. I reckon my examples and visualisations were pretty practical and relatable but still you need to say those sentences out loud. Later I’ll tell you about your homework I’ve prepared for you, but first the sentences. Listen and repeat.

She must be on the bus.

It must be hot. 

You must be tired. 

You must be kidding me.

It must have rained at night.

I must have seen this film before.

This can’t be true.

He can’t be a doctor.

You can’t be serious.

Ha can’t have been there, he didn’t have the key.

He can’t have been a doctor.

I can’t have bought them yesterday. 

I must have forgotten about eggs. 

Well done! I hope these modal verbs will be clear for you from now on and you’ll be able to use them with confidence. Thank you so much. Go to your inbox and grab the worksheet, that’s your homework. If you’re not a member download the worksheet at teacherola.com/104 and become one. It’s free. It’s worth subscribing because in my emails I give you exclusive content and I think it’s worth reading. 

Thank you for listening, and please share this episode with just one person. Your friend, your family member. Let’s spread the message. No matter what, you can start speaking English fearlessly. There’s zero doubt about it.

I’ll see you here next Wednesday. We’re going to talk about comfort zone. Happy learning. Take care! Stay fearless and say it out loud. Bye!