English Grammar

May and Must – Expressing Probability

Hiya. Probability. The chances of a happening. Sometimes you want to communicate what you think about the probability of a situation: the probability – or the chances – of that happening.

Let’s dive into it, shall we?

Look at this sentence:

I’m not quite sure but I think she is at home. 

Is she at home? Maybe. I’m not absolutely sure about that. So, the probability of this situation (her being at home) is rather low. It’s a low-probability situation.

Now look at this one:

I’m certain (I’m sure) that they are playing a video game right now.

Are they playing a video game? I think so. I’m quite positive about that. So, the probability of this situation is high. It’s a high-probability situation.

In English there are a few words to show probability of a situation or a happening, and in this video we’re going to examine 2 of them. These words are: ‘may’ and ‘must‘.

Look at the following sentences. Here are some low-probability situations:

I’m not quite sure, but I think they are on their way home from school.

They may be on their way home from school.

I’m not quite sure, but I think my dad is having a rest at the moment.

My dad may be having a rest at the moment.

I’m not quite sure, but I think they often drink beer on Friday evenings.

They may drink beer on Friday evenings.

I’m not quite sure, but I think our relatives (family members) will arrive a bit later than expected.

Our relatives may arrive a bit later than expected.

Now let’s see some high probability situations:

I’m sure the kids are still in bed.

The kids must still be in bed.

I’m certain that the neighbour’s boys are riding their bikes in the street right now.

The neighbour’s boys must be riding their bikes in the street right now.

I’m sure that my sister stops at that café on her way to work.

My sister must stop at that café on her way to work.

I’m certain that my brother will love this present.

My brother must love this present.

Good. Now here is the tricky part. Both ’may’ and ’must’ are non-past modals. It means that they don’t have past forms. Basically, they refer either to the present or to the future. So how can you express your certainty – or uncertainty for that matter – about a past event?

Here is how:

I’m not quite sure, but I think they were at the cinema last weekend.

They may have been at the cinema last weekend.

I’m not quite sure, but I think my sister has already landed.

My sister may have already landed.

I’m not quite sure, but I think Tom bought this LED TV a week or so ago.

Tom may have bought this LED TV a week or so ago.

I’m not quite sure, but I think the kids were telephoning while you were out shopping.

The kids may have been telephoning while you were out shopping.

I’m not quite sure, but I think Mrs White has been teaching for like 45 years now.

Mrs White may have been teaching for like 45 years now.

I’m sure I’ve seen him before.

I must have seen him before.

I’m certain that the neighbour mowed the lawn a few days ago.

The neighbour must have mowed the lawn a few days ago.

I’m sure that some of the employees were posting photos on Facebook while the boss was giving his presentation.

Some of the employees must have been posting photos on Facebook while the boss was giving his presentation.

I’m certain that Mr Green has been repairing cars since he was 14.

Mr Green must have been repairing cars since he was 14.

That’s all there’s to it. I think you get it now. See you next time.