Презентация на тему Modal verbs of ability : can and may 

are used to express two different types of ability:

open possibility, generally expressed by forms of the modal verb Can, and authority or potential ability, generally expressed by forms of the modal verb May. These two verbs are followed by the infinitive without to.

verb can only exists in the simple present, simple past and

present perfect forms.

writer must use forms of the synonymous modal verb

"be able to" .  This modal auxiliary has all necessary tenses, as it is in reality just the verb to be followed by the adjective able. 
N.B. : Be able to does not have any progressive or continuous forms.

to do something (by confusion with may)
Can only: in the

present perfect, a past possibility. This is particularly common with negative clauses.

not" and "cannot have" They could not see the warning

sign    = They were unable to see it, for example, because it was hidden They cannot have seen the warning sign.....       = They must have failed to see it, even though it was there and visible.

verb may only exists in the simple present, and

past forms The simple past form of may is might . Might is also used in its own right as a present tense modal.

possibility) or authority to do something. Using the modal may is

frequently the same as qualifying a statement with the word perhaps.  Its past form might is most commonly found in dependent clauses, notably in reported speech. Note that a synonym of perhaps is maybe.... which is of course composed of the words may  and be.

+ have + past participle),  may is also used to express possibility

that occurred (something that perhaps occurred) in a relative past , i.e. in past time with relation to the present or to some other moment. c) Might is also used to imply remote possibility, i.e. something that could just be possible. In this sense, it is often combined with be able to.