If can be used in place of 'can' in any tense. It is more formal in speaking and more eloquent in writing. He couldn't figure it out. = He wasn't able to figure it out. He couldn't catch a flight fast enough to make it here. = He wasn't able to catch a flight... I can't swim. = I'm not able to swim. I can't go because I'm not well. = I'm not able to go because I'm sick. I can't meet you this weekend. = I won't be able to meet you.. I can't graduate if I don't pass this class. = I won't be able to graduate if...

Might help: http://www.englishclub.com/grammar/verbs-modals_can_3.htm

Here is something super important for you to remember: IF we are talking about what happened IN A PARTICULAR SITUATION, we use "was / were able to." NOT "could." a. The fire spread through the building quickly, but everybody WAS ABLE to escape. (NOT: "could escape.") P.S, The NEGATIVE "couldn't" is OK: "Sadly, they couldn't escape." Credit for this information goes to: Grammar in Use by Raymond Murphy. (The sentence "Sadly, they couldn't escape" was my idea; ALL the other information is Mr. Murphy's.)