Modals and ability
 
 1	 The present	

'Can' and 'can't' are used to talk about general ability in the present
 
	I can swim
	She can't drive
 
They are similar in meaning to 'be able to'
 
 2	 The future	

'Can' and 'can't' are not usually used to talk about the future.
We use 'will be able to' instead

	It won’t be long before we’ll be able to order all our shopping online.

However, if we are deciding now about the future, we can use 'can' and 'can’t'
 
	I can't see you tomorrow but I can on Saturday.

 3	 The past	
 
The rules for talking about ability in the past are more complex.
 
We need to differentiate between 'general ability' and ‘ability on a specific occasion’.
 
	I could swim when I was five
	She could run a kilometer when was at secondary school. (general ability)
 
	I couldn't use my phone because the battery had run down. Fortunately I saw a 
	farmhouse and I was able to call from there. (ability on a specific occasion)

In this example, you will see that for making statements about ability on specific 
occasions, we can use ‘managed to’. ‘Was able to’ and ‘succeeded in*’ are also possible. 

* more formal / literary

Note that 'couldn’t' is possible when talking about ability on a specific occasion.
 
	I could smell gas but I couldn’t find where it was coming from.

Note also,  'could ' is possible with verbs of the senses (smell, see, hear etc.) and certain other stative verbs - understand etc.