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In the beginning, English modal verbs will, can, may, must, shall all have past tense, namely would, could, might, mote, should. They also have respective subjunctive moods to indicate unreality or intention.

Then mote, the past tense of must, became obsolete; shall grew to be rare; furthermore, the subjunctive might and should dominate the past tense might and should. There is now considerable confusion about the usage of these 8 modal verbs, and Americans probably find many of them exchangeable. Let us try to sort the matter out.

Modal verb will is used to express intention and certainty:

I will finish the report tonight.

The judge will reconsider the case.

The first course will start next week.

The house will be torn down soon.

It also refers to expectation and prediction:

You will see a beautiful garden by the lake.

More people will starve to death in the war.

Sometimes it indicates a nature or habit:

This piece of sodium will explode in the water.

A cup of milk in the morning will be good for health.

These kids will play all day long without being tired.

Modal verb can is used to express ability:

Alice can speak a little German.

Dogs can actually swim.

Both can and may refer to permission:

Students can take a cheat sheet to the exam.

Teenagers may not drink alcohol.

Both can and may indicate a tendency or possibility:

It can seem out of place to eat alone in the pub.

Violence can sometimes happen in the parade.

Bob may look upset when he works overtime.

It may be true that power only leads to corruption.

Both would and could are originally the past tense of will and can:

Last Saturday, it was reported that it would rain.

Dinosaurs would have continued their reign without the asteroid.

Einstein could play the violin.

Boris Johnson could have died of coronavirus last year.

In their subjunctive senses, would can indicate an imagined situation, and could a less likely possibility. Especially, would may be used to request, and could to ask for permission:

I would love to work remotely!

You could get infected without a mask!

Would you open the bottle for me?

Could I go with you to the ball?

Both should and must can mean obligation, necessity, and insistence. Both of them may serve as past tense, but must less frequently so:

We should lower our voice in the library.

US Army should have stayed in Afghanistan.

One must bring the ticket to the airport gate.

The drug smugglers must be punished.

In addition, must is used for speculation with greater certainty:

Something must be wrong with the Large Hadron Collider.

Amelia Earhart must have died in her last flight.

For speculation with less certainty, use may or might. But might may only be used as past tense. It is supposed that might should hint a smaller likelihood, but may and might have come to be largely exchangeable:

The pandemic may get worse by this winter.

The baby might have drowned if nobody had been there.

Beware that Eve might be eavesdropping now.

For permission or speculation, I prefer may, because can is too much overloaded; the past tense might is somewhat rare, so it is better to substitute could for that sense. It is also clearer to replace the past tense of must for necessity with should. Generally in present tense, except in a direct quote, I suggest that whenever possible, we stick to the present forms will, can, may rather than the subjunctive forms would, could, might to make tense more obvious. Reserve would have for counterfactual statements, should have for counterfactual obligation, and must have, could have, might have for counterfactual speculation in decreasing certainty.

Here I summarize my suggestions in each case, which may help to avoid some ambiguity. Of course, one doesn’t need to follow them at all times, not even myself.

❧ August 24, 2021