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 use the commercial equipment without training?
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.
Example:
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.
Example:
My neighbors have been able to purchase some real estate.
The customer has been able to make the minimum payment so he will not be penalized.
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:
The interest payments were able to be lowered by my local branch.
Were you able to do the invoicing for the credit department?
'Could' and 'was / were able to' are also used to express a general ability in the past in the positive and negative forms.
Example:
He was able to rely on savings banks in the past.
The bank could issue a voting share report when it was still a savings and loan.