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 pay using a gift card?
Are we able to estimate the withholding tax or does the figure need to be exact?
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.
We can write off that accrued liability.
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.
Susan has been able to replace her lost debit card.
We will be able to attend the shareholders' meeting.
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.
We were unable to clinch the commercial account deal with the oil company.
Many leading banks were able to reduce their lending rates.
'Could' and 'was / were able to' are also used to express a general ability in the past in the positive and negative forms.
John was able to tackle bank clearance problems when he was the manager.
The mortgage amortization could have been discounted during its first five years.