<p>Can somebody explain the difference between the two for me? I know that "can" is usually used to express ability, and "could" is used to express possibility. But sometimes we also use can to express possibility, as in "learning a second language can be hard". So when do we use "could" and when do we use "can"?</p>
<p>i think could is past tense--- consistent with 'would and should' i think..
can is present, with 'may'</p>
<p>^I'm with Ren the SAT'er here.
Present past
Can could
Shall should
May might
Will would
These words do have other functions, but this is a basic rule.</p>