Have to and must

Both 'have to' and 'must' express obligation, but they are used differently. 'Must' often shows a speaker's strong opinion or internal obligation, while 'have to' usually describes external requirements or rules. 'Have to' can be used in different tenses; 'must' has limited tense forms. Negatives and prohibition use different structures.

• 'Must' = strong obligation or logical deduction (speaker-focused).
• 'Have to' = external obligation or rules (situational).
• Use 'had to' for past obligations and 'don't have to' for lack of necessity.
• 'Mustn't' = prohibition; different from 'don't have to'.

Focus on who defines the obligation and the tense:

• 'Must' = internal or speaker-based obligation or a strong recommendation (use: must + base verb).
• 'Have to' = obligation imposed by rules, other people, or situations (use: have to + base verb; past = had to).
• Negative differences: 'don't have to' = not necessary; 'mustn't' = prohibited.

Remember: use 'had to' for past obligations and avoid adding 'to' after 'must'.

Watch for these universal errors learners make with 'must' and 'have to':

• Adding 'to' after 'must' (wrong: 'must to go').
• Using 'must' in the past (wrong: 'I musted'); use 'had to' instead.
• Confusing 'mustn't' (prohibition) with 'don't have to' (not necessary).
• Incorrect word order with modal verbs or adding extra auxiliaries (wrong: 'Do must I').
• Using negative contractions incorrectly (e.g., 'must not' vs 'mustn't') without understanding the nuance.

If unsure about tense or past forms, prefer 'have to' → 'had to' for past obligations.