We use the perfective will have when we are looking back from a point in time when something will have happened.

By the end of the decade scientists will have discovered a cure for influenza.
I will phone at six o’clock. He will have got home by then.

or looking "back" from the present:

Look at the time. The match will have started.
It’s half past five. Dad will have finished work.

We use would have as the past tense form of will have:

I phoned at six o’clock. I knew he would have got home by then.
It was half past five. Dad would have finished work.

We use would have in past conditionals to talk about something that did not happen:

If it had been a little warmer we would have gone for a swim.
He would have been very angry if he had seen you.

Exercise

Sir, I met my friend at a place where my another friend did not come but we had invited him, then we went to his home and after reaching there, I saw him doing his home work and I said " oh I thought you 'were or would be' sleeping But You are doing your home work, Nice." Now Suppose I did not go to his home, the what would be the best way to predict about him in present from these two options only "He would be sleeping or He would have been sleeping" and We had invited him yesterday but he did not come but I don't know why yet so what should I use for supposition from these two only "He would be sleeping yesterday or He would have been sleeping yesterday" and what is the difference ?

Sir, I met my friend at a place where my another friend did not come but we had invited him, then we went to his home and after reaching there, I saw him doing his home work and I said " oh I thought you 'were or would be' sleeping But You are doing your home work, Nice." Now Suppose I did not go to his home, the what would be the best way to predict about him in present from these two options only "He would be sleeping or He would have been sleeping" and We had invited him yesterday but he did not come but I don't know why yet so what should I use for supposition from these two only "He would be sleeping yesterday or He would have been sleeping yesterday" and what is the difference ?

Hello SonuKumar,

I'm afraid these kinds of intricate very specific questions based on particular imagined (and unusual) contexts go beyond what we can address on LearnEnglish. These kinds of questions require long nuanced explanations and are questions you need to ask your own teacher. We can't act as your personal teacher here as we have many thousands of users to help. As we've said in the past, we're happy to provide what help we can, particularly in terms of any uncertainties you have about the material on our pages, but we can't offer what is effectively private tuition through the comments sections.

Best wishes,

Peter

The LearnEnglish Team

Sir, I met my friend at a place where we had invited another friend but He did not come and I said "He would have been sleeping" Now I could have also used here 'Must''ve been for probability, may've been and might've been for possibility. But I used would've been for possibility and expectation, Could I also use here 'could've been for possibility and is the structure 'would've been' used for expectations and possibility rather than conditionals sentences ?

Hello SonuKumar,

Assuming that you were saying this sentence to your friend in the moment your other friend didn't show up, I'd say something like 'He must be sleeping' (if I feel somewhat certain) or 'He might be sleeping' (if I'm not sure, i.e. consider it possible). 'will be sleeping' is also possible, with a meaning similar to 'must be sleeping', but is less common. 'could be sleeping' and 'may be sleeping' mean the same as 'might be sleeping' and could also be used.

All the best,
Kirk
The LearnEnglish Team

Sir, "It's 12:pm now, my friend would be going to school" For present.
"it was 12:pm when my friend would have been going to school" for past. Could I use 'would' this for present or past possibility, expection and probability in continues forms instead of May be, might be, could be and must be. and may have been, might have been , could have been, and must have been ?

Hello SonuKumar,

We use 'will' rather than 'would' to talk about what we believe or expect to to be true in the present. Therefore your first sentence should be 'It's 12 pm now, my friend will be going to school'.

To express the same thing about the past we use would: 'At 12 pm yesterday my friend would be going to school'. You could also say '...would have been going...' with little change in meaning in this context ('would' suggest typical behaviour and 'would have' suggests a particular action).

Other modal verbs can be used, so 'should' or 'might', for example, can be used in place of 'will' and 'should have' or 'might have' can be used in place of 'would'.

Best wishes,

Peter

The LearnEnglish Team

Dear Sir
What is the structure of ' would have been ?

He looked into hiring another secretary, but it would have been too expensive
Although it had been too expensive, he had looked into hiring another secretary.

Is that both one correct?

Hello sujit,

THe second sentence is not correct.

We use 'would have' followed by a past participle (here, 'been') when we are describing things that are not real (things which are hypothetical) with a past time reference. For example, this morning I got up at 6.00 because it is a working day. At the weekend I get up at 7.00. I can think about this morning and imagine it to be different than it really was:

If it had not been a working day, I would have got up later.

You can read more about these kinds of hypothetical sentences here and here.

Best wishes,

Peter

The LearnEnglish Team

Hi..I got a doubt
Don't phone them yet, they won't have got up.
why it isn't "wouldn't" in the place of "won't"? Is it because the sentence is in present tense, the verb should be "won't"?
I'm imagining this situation which I'm not sure of whether they are sleeping or not.. So why don't we put wouldn't instead of won't? This question may appear silly. But please clarify my doubt. Thank you sir.