In this part of our lesson on modals we’re going to explore how modals can be used to express degrees of certainty. We’ll also look at the progressive forms of modals. This part assumes that you are already familiar with the English modals and their basic grammatical functions. If you are not, I recommend checking out part one of the modals lesson before continuing with this one.
Expressing Degrees of Certainty with Modals
Degrees of certainty refer to how sure you are that something is true or that something is going to happen. When you are completely sure that something is true/is going to happen, you don’t need to use a modal. You can express your certainty with just the simple present, past, and future tenses of verbs:
The roads are frozen. (Present time)
The roads were frozen yesterday. (Past time)
The roads will be frozen tomorrow. (Future time)
Degrees of Certain in the Present Time
In any case where you are not 100 percent certain, you will need to use a modal to convey your approximate degree of certainty. If you are almost, but not absolutely, certain that something is currently true, use must:
The roads must be frozen.
The weather report said that the roads would freeze. You may not have actually gone outside to check for yourself, but you can reasonable assume that this is the case.
In cases where you are less sure if the roads are indeed frozen, use may, might, or could:
The roads may be frozen.
The roads might be frozen.
The roads could be frozen.
Expressing degrees of certainty in present time in the negative follows a similar pattern. However, the negative forms of could, must, may, and might have different connotations. A simple negation still conveys 100 percent certainty:
The roads are not frozen.
The roads aren’t frozen.
Could not/couldn’t and cannot/can’t express a greater degree of certainty than the positive could does. Use these when you are about 99 percent sure that something is not true.
The roads couldn’t be frozen.
The roads can’t be frozen.
Must not is one step below couldn’t and can’t. Like the positive must, it expresses something that you have every reason to believe. It is a logical conclusion or an educated guess:
The roads must not be frozen.
Use may not and might not when expressing that you are about 50 percent or less certain that something is not true. In other words, when you are expressing a possibility:
The roads may not be frozen.
The roads might not be frozen.
The modals used for expressing certainty regarding past events (both positive and negative) correlate directly with their present time counterparts:
The roads were frozen. (You are 100 percent certain. No modal is necessary.)
The roads must have been frozen. (You are about 95 percent certain.)
The roads may/might/could have been frozen. (You are 50 percent or less certain. It is one possibility.)
The roads were not/weren’t frozen. (100 percent certain = no modal)
The roads couldn’t/can’t have been frozen. (99 percent certain)
The roads must not have been frozen. (95 percent certain)
The roads may not/might not have been frozen. (One possibility)
Using modals to express degrees of certainty about the future is slightly different. Future time modals can express both degrees of certainty and expectations for future events. As with present and past time, a simple future verb with no modal expresses 100 percent certainty:
The roads will be frozen tomorrow.
Should and ought to are used when you are almost (about 90 percent) sure that something will occur. They can also express expectations, as in the following example:
The roads should be frozen tomorrow. The National Weather Service is predicting rain and record low temperatures for the next few days.
We have every reason to believe that the temperature will drop so low that the roads will freeze over with ice. We cannot be absolutely certain, but the information supporting this expectation comes from a reliable source.
Should/ought to can also be used in the past tense to express an expectation that did not come to pass:
The roads ought to have frozen last week. It rained constantly, and the temperature never rose above freezing.
Progressive present time modals are made by combining modal + be + –ing. Progressive past time modals are constructed by combining modal + have been + –ing. There are no progressive future time modals:
Suzy isn’t answering her cell phone. She may be working.
One possible reason why Suzy isn’t taking any calls is that she is currently at work.
It’s the middle of Suzy’s shift, and she isn’t answering her phone. She must be working.
We can reasonably assume that Suzy is working right now.
Suzy didn’t answer her cell phone when I called her this morning. She may have been working.
Suzy being busy at work is a possible explanation for why she did not answer her phone earlier today.
Suzy must have been working when I called her this morning. She usually answers my calls but isn’t allowed to when she is at work.
Suzy almost always answers my calls, but she did not pick up her phone when I called her earlier today. I’m reasonably sure that the reason why was work-related.
That’s all the usages of modals that we are going to cover in this part. Next time we’ll finish our modals lesson by showing how they can be used to express ability, repeated actions, and preferences. We’ll also explain how to use modals and phrasal modals in the same sentence.