Modals of deduction and speculation

Modals of deduction and speculation are modal verbs we use to guess or conclude about situations. They allow speakers to show degrees of certainty about present or past events.

• Use modals + base verb for present/future deductions: e.g., She must be the manager.
• Use modal + have + past participle for past deductions: e.g., He might have missed the meeting.
• Different modals express different certainty levels: must (high certainty), might/may (possibility), can't (impossibility).

These structures are important in TOEIC speaking and writing to qualify statements and show professional judgment.

Remember the basic forms for deductions:

• Present/future deduction: modal + base verb → e.g., She must be the director.
• Past deduction: modal + have + past participle → e.g., They might have missed the deadline.
• Negative deduction: use can't + (have) to show impossibility → e.g., He can't be serious. / He can't have known.

Choose the modal based on how certain you are: must (strong), might/may/could (weaker), can't (impossible).

Watch out for these universal errors learners make with deduction modals:

• Using the wrong past form: write modal + have + past participle for past deductions, not modal + past simple (wrong: He must went).
• Double modals: avoid combining two modals (wrong: He might can be right).
• Confusing certainty levels: don't use must for mere possibility or might for near certainty.
• Wrong word order in questions: modal + subject + verb (correct: Could she have left?).

Focus on form (modal vs. modal + have) and the meaning (degree of certainty).