Use of modal verbs for the past tense

In principle, you cannot use modal verbs (‘may, can, will, must, would,’ etc.) in past tenses because they lack the necessary past tense forms. An exception is ‘could’ (the past form of ‘can’). To still refer to and talk about the past, there are two possibilities:

1. The use of substitute forms
2. The combination of ‘have’ and a past participle (perfect infinitive)

Both have different meanings. Compare the two sections:

Substitute forms of modal verbs for the past

The usual method of expressing the past is to conjugate the substitute form in the past tense, which every modal verb has. This form then demonstrates ability, obligation, etc. Examples:

• Present: “You must call your mother.”
• Past: “You musted call your mother.”
  • This past tense form with ‘-ed’ does not exist for this verb and is, therefore, wrong.
• Past substitute: “You had to call your mother.”

Only the modal verb ‘can’ has a form of the past, which is ‘could’. Nevertheless, it has a substitute form, which slightly differs in meaning from the standard form. The sense, however, is the same:

• Present: “I can help you.”
• Past: “I could help you.”
• Past substitute: “I was able to help you.”
• For details, see the difference in meaning between ‘can, could, be able to’.

Combination of modal verb and perfect aspect

It is also possible to combine a modal verb with the perfect form of the infinitive without ‘to’ (example: should have done). This combination expresses possibilities, obligations, assumptions, recommendations, etc. Note, however, that the meaning changes significantly compared to the present tense form. Examples:

• Present: “She must call her mother.”
  • This statement is in the present tense and expresses obligation, a must.
• Past: “She must have called her mother.”
  • The same statement in the past shows a change of meaning and represents a strong assumption.

Some modal verbs are also capable of forming a progressive form (which is the continuous form). But then, the expression refers to the present rather than to the past. Example:

• “You must have been waiting forever.”
  • Here, we have a relation to the present. For details on this form, see modal verbs in the progressive.

Conjugation rule and forms of modal verbs in the perfect

The conjugation is relatively simple, as the forms stay the same in all grammatical persons in singular and plural (I, you, he, she, it, we, they). For negation, ‘not’ is inserted after the modal verb (example: could not have seen).

Conjugation rule

Modal verb + auxiliary verb ‘have’ + past participle of the main verb (perfect infinitive)

Conjugation table and forms

Note that there are some modal verbs for which a combination of ‘have’ and participle is not possible or makes little sense.

Modal verb	English	Meaning / word sense
can	can’t have done	strong assumption
could	could have done	possibility
may	may have done	possibility (probable)
might	might have done	possibility (improbable)
must	must have done	strong assumption
shall	–	–
should	should have done	recommendation/advice
will	will have done	future form as the future perfect
would	would have done	form of possibility as the conditional perfect
ought to	ought to have done	recommendation

• With ‘can’, the expression makes sense only as negation.
• Information: ‘will’ and ‘would’ do not express modality but the future or conditional.

Explanations related to the ‘Perfect of modal verbs’

The following explanations refer to the topic ‘English modal verbs in the perfect tense of the past’ and may also help you:

• Perfective aspect
• Use of the present simple
• Use of the past simple
• Auxiliary verbs