Modal verbs are words that help us express things like necessity, ability, and how likely (or not) something is to happen. We can also use them to make a request, give an instruction, ask permission, and offer advice. They can be used in situations both real and hypothetical, and are regarded as auxiliary verbs - as they support and work in tandem with a main verb in its infinitive form.

These are the most common modal verbs:

Can, could

May, might

Shall, should

Must, have to

Will, would

These are the categories of use they most commonly fall into:

Certainty

Ability

Permission

Necessity

Requests and instructions

Advice

Certainty:

There are four different degrees of certainty within this category: Possibility, Probability, Complete Certainty, and Impossibility.

Possibility refers to something that has the potential to be true, and is expressed by the modal verbs can, could, may, and might.

Probability refers to something that has more than just the potential to be true - it is likely to be true. Probability is expressed by must and should.

Certainty refers to something that is definitely true, and is expressed by will and shall.

Impossibility refers to something that is definitely not possible, and is expressed by can't/cannot, couldn't/could not, and won't/will not.

Can is used to make a general statement about the possibility of something. It has a higher level of certainty than could, may, or might, as it is usually used when referring to something that is true occasionally.

For example, let’s say you drive to work every day and that when you do there is usually no traffic, though occasionally the traffic is bad. In that case you could accurately say, “Traffic on the way to work can be bad.” Using can makes it clear that the traffic is not certain to be bad, nor is it likely or probable to be - it is just something that you know to be a possibility.

Could, may, and might also express possibilities, but they are more hypothetical in nature. For example, if you were to say, “The traffic might be bad today”, you are essentially stating that you don’t know it will be bad with any degree of certainty - you are merely speculating that it is a possibility.

Could, may, or might can also be used to refer to a possible situation in the future or the past in a specific way.

Let's start with an example of its use in the future: "I saw the grey clouds gathering overhead and thought that it might rain later today." That statement is specific, referring to that particular day; and it expresses a possibility without a great deal of certainty. However, if we change the statement to make it less specific and more general, we must use can instead: "Grey clouds can lead to rain." The difference here is that just because there are grey clouds gathering, it does not mean that you can claim with any degree of certainty that it will rain on that particular day. However, if you are referring to grey clouds in general, you then use can to express the known possibility that they occasionally lead to rain.

When dealing with possibility in the past tense, we use could, may, or might. The correct modal verb to use differs depending on whether you are referring to something that was at one time occasionally possible, or only hypothetically possible. If you are stating that something was occasionally possible in the past, you only use could. That is because, in this context, could is used as the past tense of can, and so follows its rules of usage as described above. For example, “Before the war ended, living there could be dangerous”. That sentence implies that though it wasn’t always dangerous to live there, it was dangerous occasionally.

However, if we are referring to a hypothetical possibility in the past, we use may, might, or could, followed by have. For example, “I shouldn’t have walked along that busy road. I could have been hit by a car!” This sentence recognises the potentially dangerous situation you were in, allowing you to speculate as to what could have happened - a hypothetical possibility.