Speaking about Abilities
Can - Be Allowed to
'Can' is used to ask for and give permission. In the simplest sense, we use 'can' to request something. However, at other times 'can' expresses permission to do something specific. 'May' is also used in a more formal sense. In American English, 'May I ...' is considered the correct formal form to ask for permission.
'To be allowed to' can also be used to ask for and give permission. This form is more formal and is commonly used for rules and regulations.
Examples of Asking Permission
Can I fill out this questionnaire later today?
Are we allowed to examine your record keeping practices.
Can - To Be Able To
'Can' is commonly used to express the ability to do something. This usage is similar to the form 'to be able to do something'. Both these forms are acceptable, although 'can' is more commonly used.
He can lead an international humanitarian intervention.
Jon is able to explain the effects of racism.
To Be Able To - Future and Perfect Forms
There is no future or perfect form of 'can'. Use 'to be able to' in all future and perfect tenses.
Peter has been able to find a new job with a relief-oriented versus development-oriented organization.
Terry Gilliam has been able to research the subject for his film.
Special Case of the Past Positive
Use 'was / were able to' when speaking about a specific event in the past in the positive form. Both 'can' and 'to be able to' are used in the past negative to speak about specific events.
Jane was able to reform the regulations and allow for greater private control.
Jenny was able to explain the issues during her presentation.
'Could' and 'was / were able to' are also used to express a general ability in the past in the positive and negative forms.
Bob was able to bring local action to bear on international issues when he was a community organizer.
The trainer could discuss a wide variety of interpersonal skills.