Modals for Degrees of Certainty

Modals are auxiliary verbs that express different degrees of certainty, ranging from strong certainty to possibility or even impossibility. When used to indicate certainty, modals help convey the speaker's level of confidence about a statement or situation. Below is an explanation of modals that express degrees of certainty, their comparisons, and example sentences.

Must – Strong Certainty

"Must" is used to express a high level of certainty. It is used when the speaker is almost sure that something is true, based on evidence or strong logic.

1. She must be at home, because her car is parked outside. (The speaker is almost certain that she is at home due to the visible evidence.)
2. He must have forgotten about the meeting. (The speaker is quite certain, based on his behavior or some other reason.)

Explanation: "Must" shows that the speaker is confident about a fact or conclusion, often based on observable clues or logical deduction. It indicates a very high degree of certainty.

Can't / Cannot – Strong Certainty (Negative)

"Can't" or "cannot" is the negative form of "must." It is used to express strong disbelief or to indicate something that is definitely not true.

1. She can't be at home; her car is not in the driveway. (The speaker is certain she is not home because there is no evidence to suggest otherwise.)
2. He can't have forgotten about the meeting; he was reminded this morning. (The speaker is very sure that he didn't forget, as he was reminded.)

Explanation: "Can't" or "cannot" is used when the speaker is very sure that something is not true. It's often based on the absence of evidence or reasoning that contradicts the statement.

May / Might / Could – Possibility (Less Certain)

"May," "might," and "could" are modals used to indicate possibility. They show a lower level of certainty compared to "must" and are used when the speaker is not sure, but believes something is possible.

1. It may rain later, so you should bring an umbrella. (The speaker thinks it is possible that it will rain, but is not certain.)
2. She might go to the party tonight, but she hasn't decided yet. (The speaker is unsure about whether she will attend the party.)
3. He could be the one who broke the vase, but I’m not sure. (The speaker suggests that there is a possibility, but does not have full evidence.)

Explanation: These modals express varying degrees of possibility. "May" is slightly more certain than "might," while "could" suggests a possibility with even less confidence. All three indicate situations where something is likely but not definite.

Can't / Couldn't – Possibility (Negative, Less Certain)

"Can't" and "couldn't" can also express a possibility in the negative, indicating something is unlikely or impossible, though with less certainty than "must not."

1. She can’t be the one who ate the cake; she was on the phone all day. (The speaker is almost certain, but not fully sure, that she didn't eat the cake.)
2. He couldn’t have known about the surprise party. (The speaker believes it is highly unlikely that he knew, though not impossible.)

Explanation: "Can't" and "couldn't" in negative form express something that seems improbable, but the speaker is not absolutely certain.