Using Modals of Probability in the Present, Past, and Future

When we use modals of probability, we are not stating absolute facts. Instead, we are interpreting the world based on the evidence we have and our own reasoning. Depending on the time frame — present, past, or future — the structure changes significantly. The core social and interpretive function remains the same: we are showing how certain (or uncertain) we are about a situation, from the speaker's point of view.

1. Present Probability (Now)

To describe our present deductions about what is happening right now, we use a modal followed directly by the base form of the main verb (or be + V-ing for actions in progress right now).

• Must + base verb – near 100% certainty (logical conclusion).
  “She must be at home right now.” (I can't see her, but her car is here.)
• May / Might / Could + base verb – possibility (50% or less).
  “He might be sleeping; I won't knock loudly.” (I am guessing based on the quiet house.)
• Can't / Couldn't + base verb – strong negative deduction (impossible).
  “That can't be the answer — it's too simple.” (I firmly reject that interpretation.)
• Should / Ought to + base verb – logical expectation (what we reasonably suppose).
  “The meeting should be over by now.” (My expectation based on the schedule.)

2. Past Probability (Before Now)

To make deductions about completed past actions, we must shift the time reference. The structure becomes: modal + have + past participle (V3) (or have been + V-ing for ongoing past actions). This is the only way to "move" the modal into the past, as modals themselves have no past tense for probability.

• Must + have + V3 – strong certainty about a past event.
  “They must have left early; the office is empty.” (My conclusion about the past based on the present empty office.)
• May / Might / Could + have + V3 – speculation about a past possibility.
  “She might have missed the train.” (I am considering a possible past reason for her absence.)
• Can't / Couldn't + have + V3 – strong certainty that a past action did NOT happen.
  “He can't have finished the project already — it was too complex.” (My reasoned rejection of the past event.)
• Should / Ought to + have + V3 – expectation about a past event that may or may not have occurred (often implying something went wrong).
  “The package should have arrived yesterday.” (I expected it, but I'm checking if it actually did.)

Note on continuous past: “She must have been waiting for hours.” – emphasises the duration of the past probability.

3. Future Probability (Later)

Predicting the future is inherently uncertain. For future probability, we generally use modals like will, should, may, might, and could followed by the base form of the verb. Note that must is rarely used for future prediction because we cannot be logically certain about things that haven't happened yet.

• Will + base verb – strong prediction (near certainty based on known facts or natural order).
  “The sun will rise at 6 AM tomorrow.” (Factual certainty.)
• Should + base verb – reasonable, expected future outcome.
  “We should arrive by noon if the traffic is light.” (Logical expectation.)
• May / Might / Could + base verb – future possibility (less certain).
  “It might rain later, so take an umbrella.” (I am speculating about a future condition.)
• Can't / Couldn't (used rarely for future) – we usually use can't for present/past logic, but it can appear in conditional futures: “That can't happen in the future if we prevent it now.” (Interpretive opinion on the impossibility of a future event.)

Comparison Table: Probability Across Time

Special Note: Continuous Forms (Probability + Duration)

Just as we saw with the Perfect Continuous tense, we can combine modals of probability with continuous forms to emphasise the duration of a speculated action:

• Present continuous: “He must be sleeping right now.” (Action ongoing now.)
• Past continuous: “She might have been crying — her eyes are red.” (Speculating about an ongoing past action.)
• Future continuous: “They will be working on the project at this time tomorrow.” (Predicting an ongoing future action.)