The present progressive form of a modal auxiliary is composed of a modal auxiliary + be + verb form ending in -ing. The meaning consists of the meaning of the modal auxiliary and that of the present-tense progressive form of the verb.
In the example below, the speaker wonders (may) whether Sophie is resting.
- We should probably come back later. Sophie may be resting.
In the example below, the speaker believes (must) that the store is closing.
- The lights inside are being turned off. The store must be closing.
The past progressive form is composed of a modal auxiliary + have been + verb form ending in -ing. The meaning consists of the meaning of the modal auxiliary and that of the past-tense progressive form of the verb.
In the example below, the speaker suspects (might) that Diana was sleeping in.
- Diana wasn’t at church today. She might have been sleeping in.
In the example below, the speaker believes (must) that Paul was studying all night.
- Paul looked tired this morning. He must have been studying all night.
By using a modal auxiliary, a speaker is choosing to express a degree of certainty or uncertainty. The degree of certainty reflects how sure the speaker is of something happening or how true his or her statement might be. If the speaker is sure of something, he or she doesn’t use a modal.
- He is sick.
If the speaker wants to express a strong degree of certainty, he or she uses must.
- He must be sick.
If the speaker wants to express a weak degree of certainty, he or she uses may, might, or could:
- He may be sick.
The distinction between two forms of modal auxiliaries needs to be clarified: used to and be used to. Used to expresses a “habitual past,” an activity or a situation that existed in the past but no longer exists. It is formed by using used to + base form of the verb.
- Alfred used to work for IBM.
- My family used to vacation in Maryland.
Be used to is equivalent in meaning to “be familiar with” or “be accustomed to.” Both be used to and be accustomed to can be followed by a gerund (a verb form ending in -ing), a noun phrase, or a pronoun.
- Kevin grew up in Alaska, so he is used to living in cold weather.
- I think I’m finally used to working nights.
- He wasn’t used to such rude behavior.
- I’m finally used to it.
The modal auxiliaries would and used to are interchangeable when they express a habitual past.
- My brother and I used to go skiing every morning.
- My brother and I would go skiing every morning.
However, when used to expresses a situation or state of being in the past, it cannot be replaced by would. This occurs most frequently with the verb be. The modal auxiliary would can only be used to express a recurring action in the past.
- I used to be a firefighter. (Would cannot be used.)
- Didn’t you used to be a flight attendant? (Would cannot be used.)