'May have' vs. 'Can have'

[truebytes]

Dear Friends,

This is my first question in this forum

I am very confused about the usage of 'may have / can have'. I checked almost all the grammar books. All the grammar books says that 'may have' used to say about a past action. My doubt is about the following sentences.

'You may have got thinner by the time you get back'
'You may not have reached there by morning'

I feel this is about a future thing.

Same thing about 'can't have'. Please tell me the meaning of the following sentence.

'We can't have finished the job by next sunday'

'I can't have raised the money by then'

'You can't have kept it a secret for a long.

Kindly help me.

[diverhank]

May I ask where these examples come from? They seem wrong to me but I can't put my fingers on them. Can you give these in a context (like in a paragraph)?

[truebytes]

[Alan]

Hi Truebytes,

There seems to be some kind of confusion here. I think the best way is to look at the underlying meanings of 'may' and 'can'. 'May' has the sense of possibility or permission and 'can' the sense of possibilty together with ability.

We say: I may swim = I have permission to swim or I possibly will swim but I haven't decided yet.

We say: I can swim = I am able to swim or I have the ability to swim.

If we then change the infinitive 'swim' into a perfect infinitive (have swum), the meanings change.

We say: I may have swum along that river when I was younger = It is possible that I did this when I was younger - I can't really remember.

We do not say: I can have swum because this makes no sense as it suggests uncertainty and certainty at the same time.

It is possible to say: I could have swum, meaning it would have been possible but I didn't do it or I can't have swum or couldn't have swum, meaning it's not possible that I did or it would not have been possible if I had tried but I didn't try.

Alan
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English as a Second Language
You can read my ESL story Too Many Words

[truebytes]

Dear Alan,

Many thanks for the help.

Still I am confused with the below sentences. Can't we use 'may' & 'can' in future perfect ? How can we tell the thing that may happend in the future which we are not sure.

'You may have got thinner by the time you get back'
'You may not have reached there by morning'

'We can't have finished the job by next sunday'
'I can't have raised the money by then'
'You can't have kept it a secret for a long.

Please help me

[Alan]

Hi,

These two are possible:

[truebytes]

Dear Alan,

Thanks very much for the help.

It was being confused me for some time.

Thanks again.

With warm regds
Benny