# Why modals use present perfect not simple past

I teach ESL for a community college. We just did a unit on "modals of probability" (can't, couldn't, must, should, may, might, could). For the simple present tense, we use this formula "modal + base verb" (He must have at least enough money to get a soda.) However, for the past tense, the formula includes what looks like the present perfect: "modal + have + past participle". Is it the present perfect masquerading as the simple past? One website I saw called this formula the "past infinitive". I've never heard of that, have you? Thanks everyone, Jennifer Rueda (Clackamas Community College - Oregon)

• Syntactically, "must" is a present tense modal aux verb, so it follows that "He must have at least enough money" is a present tense clause. But the modal "could", for example, is a past tense form, thus "He could have enough money" is a past tense clause. The perfect is a past tense that can combine with preterite and present tense but can also occur in clauses without inflectional tense. "He must have had enough money" is thus just as much perfect tense, as "He had had enough money" is. Jul 5, 2018 at 6:47

1. He may go. He might be. He could leave.
2. He may have gone. He might have been. He could have left.

That second pair are NOT using present perfect. Present prefect would be something like

1. He has gone.

All modals take a verb in the infinitive, which means it cannot be present anything.

If you want to call have gone or have been a perfect infinitive or an infinitive perfect, you’re certainly welcome to, but the important thing is that it’s got to be an infinitive to take a modal.

Since there’s no finite verb remaining here to carry the tense, sometimes periphrastic phrases are used.

1. He may go. He may have gone.
2. He is able to go. He was able to go. He had been able to go.

So now you can work around the problem in 6 via 7:

1. If he couldn't have left, he wouldn't have left.
2. If he hadn't been able to leave, he wouldn't have left.
• Thanks, Tchrist. It looks like I labeled it "present perfect" when it's actually an perfect infinitive, something I've never heard of. Thanks for the info. Jen Jul 7, 2018 at 2:10

There is a comprehensive discussion and analysis of "have" after modals in McCawley's text, and much of it is available on line at this reference. Look at page 220 and the following discussion, or search on "tense replacement". In a nutshell, "have"+en replaces past tense in non-finite contexts.