Can
Can generally refers to one’s ability to do something. By definition, it means:
to be able to; have the ability, power, or skill to (Dictionary.com)
When using can, you will most likely be referring to this ability, power, or skill in the present or future tense. For example, “I can go to the store after dinner.” This is interchangeable with “I have the ability to go to the store after dinner.”
Could
Could can function in a couple different ways. The first is the past tense of can, which is the primary form of functioning. “In 1985, one could dance like a robot without being laughed at.” Since we’re discussing 1985, could is appropriate here. “In 1985, one can dance like a robot…” doesn’t really make sense.
The other use of could is to establish a level of doubt, tentativeness or politeness. If someone asked you, “Can you do this for me?” or “Could you do this for me?” which would sound more polite and formal? Clearly the latter. Let’s look at an example of establishing doubt:
I can go to the store later.
I could go to the store later.
When you say “I can” you are just establishing that you have the ability to do so. When you say “I could” you establish that, yes you have the ability, but it’s only a possibility that you will go.
Final Thoughts
Can is generally present tense and is used to show you have the ability to do something. Could is used to be the past tense form of can, used to establish doubt, or used to be more formal.
