TOEIC® Certainty and Uncertainty Modals: Avoiding the Pitfalls (must vs should)

Modals of certainty and uncertainty (must, might, could, can't, should, may) express the degree of probability that an event is true or will occur.

In the TOEIC®, they appear mainly in Part 5 in professional contexts: predictions about a project, assumptions about a situation, logical deductions.

The classic trap? Confusing must (strong certainty = logical deduction) with should (medium probability = reasonable expectation). Exam designers often trick candidates on this nuance in emails and reports.

Why the TOEIC® loves setting traps with these modals

In a professional context, the difference between "it's certain" and "it's probable" completely changes the meaning of a message.

An email saying "The report must be ready" (deduction: it is definitely ready) is very different from "The report should be ready" (expectation: it is expected to be ready, normally).

The TOEIC® plays on this in Part 5 with sentences where several modals seem possible. Context then becomes your only ally: time clues, logical keywords (because, since), the general tone of the sentence.

We see it all the time with our candidates: even those who know the theory freeze under pressure. They choose "must" out of reflex (because it sounds stronger), when the context requires "should" or "might". Remember this well: the modal must match the degree of certainty suggested by the context, not your intuition.

The Certainty Scale: From Most Certain to Least Certain

Here is how to rank them from most certain to least certain:

Frequent Part 5 Trap: The TOEIC® offers "must" and "should" as options in a sentence where a subtle clue (like "probably", "I think", "usually") mandates "should".

Example:

• "Based on previous trends, sales _______ increase next quarter."
  → Answer: should (reasonable expectation based on habit, not absolute factual deduction)

Mistakes That Cost Points Unnecessarily

We see it repeatedly with candidates we coach for the TOEIC®: three confusions come up all the time.

Mistake 1: Translating "devoir" (must/have to) as "must" systematically

In French, "Il doit être en réunion" can mean obligation OR deduction. In English, it's different:

• Deduction = "He must be in a meeting." (I deduce this from the facts)
• Obligation = "He has to attend the meeting." (It is required)

Mistake 2: Using "must" for the future

"Must" expresses a deduction about the present or past, rarely the future. For the future, prefer "should" or "will probably".

• Incorrect: "The project must finish on time." (strange deduction about the future)
• Correct: "The project should finish on time." (reasonable expectation)

Mistake 3: Mixing up "may" and "maybe"

"May" is a modal (+ base verb without TO). "Maybe" is an adverb.

• Incorrect: "She maybe leaves early."
• Correct: "She may leave early." She might leave early.

TOEIC® Traps to Anticipate Table

Typical Part 5 Example:

"Mr. Lee hasn't replied to three emails. He _______ have seen them yet."

A) must B) should C) can't D) might

Analysis: "hasn't replied to three" = observed fact → logical impossibility that he has seen them.

Answer: C) can't

Decision Checklist: Which Modal to Choose?

Use this 3-step method when you are hesitating in Part 5:

Step 1: Identify the Degree of Certainty

• Are there concrete proofs mentioned? (because, since, obviously) → must / can't
• Are there words like "probably", "I think", "usually"? → should / may
• Is the tone speculative or cautious? → might / could

Step 2: Check the Tense

• Present/Past + deduction → must / can't
• Future + expectation → should
• Future + simple possibility → may / might

Step 3: Eliminate Intruders

• "Mustn't" = prohibition (almost never the answer for this type of question)
• "Could" in an assertion = low possibility (often a distraction)

The Negative Form: Be Aware of the Meaning

"Mustn't" and "can't" do NOT mean the same thing:

• mustn't = prohibition ("You mustn't smoke here." You must not smoke here.)
• can't = logical impossibility ("She can't be the CEO. She's 19." She cannot possibly be the CEO. She is 19.)

In the TOEIC®, "can't" appears in negative deductions; "mustn't" is rare (outside of regulation/prohibition contexts).

Even among candidates who already have access to a preparation platform through their school, the confusion between "must" and "should" comes up very frequently in Part 5. The reason? These tools emphasize the rule, but rarely the reflex to adopt under pressure.

Ready to practice?

Now that you have mastered the modals of certainty and uncertainty, it is time to practice.

On Flow Exam, you can practice directly on the Modals topic in Part 5, with thousands of questions in the same format as the official TOEIC® ones. So if you struggle with this topic, you will never make the same mistakes again.

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