"must + [verb]" past simple
Is there a way to use must in past simple?
Present simple: You must run.
Past perfect: You must have ran.
Past simple doesn't work: You must ran.
Not even with another word to indicate past (since must is ambiguous): I thought you must ran. (wrong)
Solution 1:
had to
I must go. (present)
I had to go. (past)
Solution 2:
Of the nine standard modal verbs in English, only eight of these are inflected for tense as four pairs:
| Present Tense | Past Tense |
|---|---|
| will | would |
| shall | should |
| can | could |
| may | might |
And these naturally pair like this:
| Present Tense | Past Tense |
|---|---|
| Today, I think I will go. | Yesterday, I thought I would go. |
| I think I can get to it today. | I thought I could get to it yesterday. |
| It looks like it may rain today. | It looked like it might rain that night. |
But the ninth modal, must, has no inflectional pair to use with either possible tense. It works best for a nonpast situation only:
| Present Tense | Past Tense |
|---|---|
| Today I have decided that I must go shopping. | Yesterday I had decided that I must go shopping. |
| She must finish her project today. | — |
And you cannot use must have either, because that is an epistemic use making a prediction of how the world is not a deontic use stating how the world needs to be but is not:
| Present Tense | Past Tense |
|---|---|
| Surely she has gone home already. | Surely she had gone home already. |
| I think she must have gone home already. | I thought she must have gone home already. |
| I think she will have gone home already. | I thought she would have gone home already. |
Therefore you must replace must with its periphrastic equivalent that employs a tensed auxiliary to carry the tense. Either to have to or to be to works fine for this:
| Present Tense | Past Tense |
|---|---|
| She has to finish this today. | She had to finish that yesterday. |
| She is to finish this today. | She was to finish that yesterday. |
| I think she has to have gone home already. | I thought she had to have gone home already. |
| I think she will have to finish tomorrow. | I thought she would have to finish tomorrow. |
When replacing must with one of those two periphrastic equivalents, the to be to version is more likely to be used epistemically for a prediction, while to have to is more apt to be used deontically for an obligation.
But this is not guaranteed. For example, these are all predictions not obligations:
- This must be John at the door now.
- This has to be John at the door now.
- That had to be John at the door back then.
- That must have been John at the door back then.
- That had to have been John at the door back then.