Modal verbsofdeduction

may / might /could : you think something is possibly true

can’t : you are sure something is impossible / not true

must : you are sure something is true

Is Sally at home now?

We use indicative tensesto say what we know

Sally is at home now.

I know

We use modals when wesuppose, deduce, imagine

Sally must be at home now.

I suppose

The choice of modal verb says how certain we are

Sally

maymightcouldcan’tmust

be at home now.

Sally

maymightcouldcan’tmust

be at home now.

it’s possible

Sally

maymightcouldcan’tmust

be at home now.

I’m sure she isn’t(even if I don’t

know)

Sally

maymightcouldcan’tmust

be at home now.

I’m convinced (even if I don’t

know)

Notice the opposites!!

Sally must be at home now.Sally can’t be at home now.

Modal verbs of deductionhave a continuous form

Something

maymightcouldcan’tmust

be working.

Use the continuous infinitive after the modal

continuous infinitivebe working

be speakingbe thinking

etc.

But, be careful!!

Sometimes the meaning changes depending on whether

you use the simple or the continuous infinitive

e.g.

He can’t speak French.He can’t be speaking in French.

e.g.

He can’t speak French.He can’t be speaking in French.

he doesn’t know how

e.g.

He can’t speak French.He can’t be speaking in French.

I don’t believe he’s doing it

Modal verbs of deductionhave a past form

Something

maymightcouldcan’tmust

have worked.

Use the perfect infinitiveafter the modal

perfect infinitivehave workedhave spokenhave thought

etc.

But, be careful!!

Other meanings of the modals don’t use the perfect infinitive

e.g.

He couldn’t speak French.He can’t have spoken in French.

e.g.

He couldn’t speak French.He can’t have spoken in French.

he didn’t know howwhen he was

younger

e.g.

He couldn’t speak French.He can’t have spoken in French.

I don’t believe he spoke in French

Let’s practise

Convert continuous modalsto perfect

and perfect modalsto continuous

continuous modal perfect modal

She might be calling.

They can’t have studied.

He must be going by bus.

She won’t have cooked dinner.

He will be wearing the jacket.

They won’t have taken their exams.

I must be dreaming it.

They may have argued.

He can’t be choosing.

You must have paid a lot.

She will be enjoying her holiday.

continuous modal perfect modal

She might be calling. She might have called.

They can’t be studying. They can’t have studied.

He must be going by bus. He must have gone by bus.

She won’t be cooking dinner. She won’t have cooked dinner.

He will be wearing the jacket. He will have worn the jacket.

They won’t be taking their exams. They won’t have taken their exams.

I must be dreaming it. I must have dreamt it.

They may be arguing. They may have argued.

He can’t be choosing. He can’t have chosen.

You must be paying a lot. You must have paid a lot.

She will be enjoying her holiday. She will have enjoyed her holiday.

Let’s put all that into practice

http://learnenglish.britishcouncil.org/es/grammar-reference/modals-deduction-present

http://www.perfect-english-grammar.com/modal-verbs-of-probability-exercise-1.html