Using Modals to Express Degrees of Certainty

There are many ways to express certainty in English. Adverbs like probably, maybe or definitely can be used. Certain regular verbs like think, believe or be sure can be used.

But one of the toughest ways (and most natural) ways to do it are using modal verbs.  What are modal verbs?  They are verbs that help express possibility, ability, permission and duty.  Here are some common examples:

  • Can
  • Could
  • May
  • Might
  • Should
  • Must

Remember these?  Yes, they are a challenge!  Probably the most challenging way to use them is for past sentences.  Here are the most common examples (PP stands for Past Participle):

  • I must have PP (I must have forgotten)
  • I may (not) have PP (I may not have turned off the light)
  • I might (not) have PP (I might have left it at the restaurant)
  • I could(n’t) have PP (I could have put it in the kitchen)

Wow!  That’s quite a few… so how can you know when you should use them, and which one you should use?

Well, let’s start with some things that are true about all of them:

  • They all express uncertainty.  In other words, they mean the speaker is not 100% sure about what he is saying.
  • They are all used to express something about the past.
  • They all use the perfect form (S-have/has-PP-O).

Here are some more examples:

  • I can’t find my wallet.  I must have left it at the restaurant.
  • No, you couldn’t have left it there.  I saw you put it in your pocket.
  • I don’t know.  I may have taken it out again when I went to the bathroom.

So are all these expressions the same?  Well… not exactly.

High Confidence vs. Low Confidence

First, I need to start by saying this: Please ignore anybody who gives you a percentage!

Lots of teachers and native speakers like to say things like “Maybe means 50%” or “Probably means 75%” or something like that.  No, no, no!  When we speak our native language, we are not calculating the percentage in our head, we are thinking about how confident we feel about the statement.

So really, all we can say is that certain words mean we feel high confidence (I think there is a good probability it is true), certain words mean we feel low confidence (I don’t think there is a good probability it is true).

So what are they?

Expressions of high confidence: must and couldn’t

Expressions of low confidence: may, might and could

Think about it this way: If someone asked you to bet money that the statement is true, would you?  If you would, then you should use an expression of high confidence.  If you wouldn’t, they you should use an expression of low confidence.

Did you notice that could and couldn’t are in different categories?  Strange, huh?  We’ll look at those in a moment.

First I’d like to talk about the difference between must and couldn’t.  It’s actually quite simple.  Must expresses a higher level of confidence than should.

Must vs. Couldn’t:

Basically, must means “I’m almost completely (100%) certain”, but couldn’t just means “I’m very certain (but not completely).

Let’s go back to the lost wallet example:

  • I know I had my wallet before we went to the restaurant because I used it in the restaurant.  After we left the restaurant we got in the car and drove home.  We both checked the car several times so it’s not there.  I must have lost it in the restaurant.  Let’s call them.
  • I think I had my wallet before we went to the restaurant, but you paid for dinner so I’m not sure.  After we left the restaurant we stopped by a convenience store on the way home.  I didn’t take my wallet out there, so I don’t think I lost it there.  I checked the car a few times so I’m sure it’s not there.  I think it’s at the restaurant, because I couldn’t have lost it anywhere else.

As you can see by a lot of the language, in both cases the speaker is highly confident, but more confident in the first case than the second.

So why is couldn’t more confident than could?  The reason is simple, but may be hard to understand:

Could is contributing one possibility among many.  Couldn’t is eliminating a specific possibility.
It is easier to eliminate a possibility than contribute one.

So what does this mean?  Well, consider these two statements:

  1. He could have forgotten to call. = There are many possibilities, and this is only one.
  2. He couldn’t have forgotten to call. = There are many possibilities, but this is NOT one.

When there are many possibilities, it is easier to be sure about which possibility is not true than which possibility is true.

Let’s take another example.  You say hello to a co-worker.  He does not say hello back.  What are the possible reasons?

  1. He’s mad at you.
  2. He doesn’t like you.
  3. He didn’t hear you.
  4. He was busy thinking about something.

Which one is true?  It’s difficult to say sometimes.

How about these possibilities:

  1. He’s an alien.
  2. He has a policy about not saying hello.
  3. He is deaf.
  4. His wife told him not to say hello to his co-workers any more.

Sure, those are possible, but do you think they are true?  It’s easier to say these are not true.

So let’s get to the expressions of low confidence.  What’s the difference?

May vs. Might

This is very easy to explain.  May is more formal than might.  That’s it.

May and Might vs. Could

These are essentially the same in this context (talking about uncertainty).  However, “could have PP” can be confusing, because it can be used other ways.

  • Could have PP is sometimes used to scold people
    • “Oh my God, why didn’t you go to the party??  You could have had so much fun!”
    • “If you just studied harder for the test, you could have passed.”
  • Could have PP is sometimes used to express missed opportunities
    • “If my co-worker hadn’t gotten sick I could have taken a day off today.”
    • “If I had some money I could have gone to that concert.”

So for this reason, be careful when using “could have”.  It can be misunderstood.

That’s all!  I hope this was helpful.  Next time you have a chance to talk about the past, try using these modal verbs!


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