> For the complete documentation index, see [llms.txt](https://viktor-8.gitbook.io/untitled/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://viktor-8.gitbook.io/untitled/english/past-modal-verbs-of-deduction.md).

# Past modal verbs of deduction

Modal verbs of deduction and speculation

We can use some modal verbs + infinitive to talk about how certain we are that something is or is not true. We can also use some modal verbs + **have** + past participle to talk about how certain we are that something was or was not true in the past.

### <mark style="color:blue;">**Must have done**</mark>

We use <mark style="color:green;">**must have + past participle**</mark> to say that we are quite sure that something <mark style="color:green;">**was true or happened in the past**</mark>.

* *I can’t find my wallet! I <mark style="color:orange;">**must have dropped**</mark> it in the taxi.*
* *You <mark style="color:orange;">**must have had**</mark>* *a real scare when you saw the crocodile.*&#x20;

### <mark style="color:blue;">**Can’t have done**</mark>

We use <mark style="color:green;">**can’t/couldn’t have + past participle**</mark> to say that we are <mark style="color:green;">**quite sure that something did NOT happen or was NOT true in the past**</mark>.

* *You <mark style="color:orange;">**can’t/couldn’t have seen**</mark> John last night. He was in hospital.*&#x20;
* *She <mark style="color:orange;">**can’t/couldn’t have passed**</mark> the test. She didn’t even open the books.*&#x20;

Note that for negative deduction we use <mark style="color:green;">**can’t**</mark> (NOT ~~<mark style="color:green;">**mustn’t**</mark>~~)

* *He <mark style="color:orange;">**can’t be**</mark> that famous.* (NOT ~~*<mark style="color:orange;">He mustn’t be that famous</mark>*~~*<mark style="color:orange;">.</mark>*)

### <mark style="color:blue;">**Could/might/may have done**</mark>

We use <mark style="color:green;">**might/may have + past participle**</mark> to say that <mark style="color:green;">**it’s possible that**</mark> <mark style="color:green;">**something was true or happened**</mark> in the past.

* *It’s been three days. They <mark style="color:orange;">**might/may**</mark>* *<mark style="color:orange;">**have finished**</mark> painting the house by now.*&#x20;
* *If they left at 9, they <mark style="color:orange;">**might/may have**</mark>* *already* *<mark style="color:orange;">**arrived**</mark>.*

Note that we use <mark style="color:green;">**might not**</mark> or <mark style="color:green;">**may not**</mark> (NOT ~~<mark style="color:green;">**could not**</mark>~~) to talk about a negative possibility.

* *She <mark style="color:orange;">**might/may not have heard**</mark> us. Knock again.*&#x20;

### <mark style="color:blue;">**Should/ought to have done**</mark>

We use <mark style="color:green;">**should have**</mark><mark style="color:green;">/</mark><mark style="color:green;">**ought to have + past participle**</mark> to talk about a situation expected to happen in the past. This form is normally used for criticism.

* *You <mark style="color:orange;">**should/ought to**</mark>* *<mark style="color:orange;">**have**</mark> already <mark style="color:orange;">**packed**</mark> your things. We’re going to be late.*&#x20;
* *He <mark style="color:orange;">**should/ought to have studied**</mark> more. Nobody fails if they study.*&#x20;