Modal verbs show how certain or possible something is. Will, must, should, may, might and could all sit in front of another verb and change how strongly the writer commits to the statement. In reports, reviews and speaking discussions, your choice of modal is part of your opinion.
This is the last lesson in grammar for clear meaning and it builds on the cause and contrast links from the previous lesson.
How do the modals rank for certainty?
Think of a scale from almost sure to only possible.
| Strength | Modal | Meaning | Example |
|---|---|---|---|
| Very high | will, must | Certain, or a strong conclusion | The shop will open at nine. The door is locked, so it must be closed. |
| Expected | should, ought to | Reasonable expectation | The parcel should arrive tomorrow. |
| Possible | may, might, could | Real chance, not sure | It may rain later. She could be in the library. |
| Very low | might not, could not | Doubtful or impossible | He might not finish in time. That could not be true. |
Two points keep the choices clear. First, must for a conclusion comes from evidence you can see. Second, can describes ability or general possibility, not a guess about a particular situation.
All modals use the base form of the next verb: must be, might go, should arrive.
Worked example
Rewrite the sentence so the certainty matches the evidence: The museum is full of people, so it is open. The sign says school groups arrive at ten, so the noise is loud.
Step 1, ask how strong the evidence is. A full museum is strong evidence that it is open. The noise is an assumption from a sign, so it is less certain.
Step 2, choose for the first claim. Strong evidence suggests must: The museum is full of people, so it must be open.
Step 3, choose for the second claim. The sign gives a reason to expect noise, not proof: so it might be loud or so it should be loud.
Step 4, choose the final wording. The museum is full of people, so it must be open. The sign says school groups arrive at ten, so it might be loud by then.
Step 5, check the verb forms. After must and might there is be, the base form.
The mistake to watch for
A frequent slip is stating a guess as a fact, or hedging a fact that is certain.
Mistaken sentence: The results are not on the website yet, so they will be late.
The writer cannot know this, yet the modal presents it as certain.
Correction: The results are not on the website yet, so they may be late. The evidence supports a possibility, not a certainty. Reverse the error too: Water might boil at 100 °C at sea level weakens a known fact. Write Water boils at 100 °C at sea level.
A second slip is adding to or an -s ending: She might to go, He can goes.
Check yourself
Try these, then open each answer.
1. Choose the most suitable modal: Her coat is on the chair and her bag is here, so she ___ be nearby. (must / might / will)
Show answer
must. Her coat and bag are strong evidence, so the conclusion is almost certain.
2. Correct: It could to rain tomorrow.
Show answer
It could rain tomorrow. No to after a modal verb.
3. A friend has not replied to your message for one hour. Write a careful sentence about why.
Show answer
She might be busy. or She may not have seen it yet. An hour of silence is weak evidence, so possibility modals fit better than must.
Where this leads next
With all five lessons done, test yourself with the mixed grammar practice set. Then continue to sentence building and paraphrase, where accurate grammar helps you restate ideas in your own words.
If you often hear yourself sounding either too certain or too vague, a teacher can rehearse the difference with you in online one-to-one English as a Second Language tuition.