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 borrow your car?
Is the contractor able to continue work on the house while the homeowner is awaiting your reply regarding financing?
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.
Example:
You can use direct deposit to wire money straight into your account.
We are able to adjust our lending rates.
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.
Example:
The issuing bank will not be able to activate the card until tomorrow.
Our clients will be able to use a credit or debit card to complete their transactions.
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.
Example:
George was able to augment his inventory of capital goods by buying in quantity during the previous fiscal year.
I was able to find out a lot about my company's credit rating this morning.
'Could' and 'was / were able to' are also used to express a general ability in the past in the positive and negative forms.
Example:
John was able to tackle bank clearance problems when he was the manager.
The gross profit could always save the company's other excesses when he was president.