"The reason you couldn’t sprint a marathon, he says, is that your body can’t supply energy rapidly enough using the aerobic system." Extracted from a BBC article. I can't understand the use of could, We use can for a possibility/ability in the present, don't we?
1 Answer
You're correct that can expresses the ability in the present tense and the phrase "The reason you can't sprint a marathon" is perfectly valid.
The sentence you've quoted is not using the past tense of to be able; as you've hinted at, it uses the conditional mood.
The meaning of the sentence in the present tense is different.
"The reason you can't sprint a marathon..." - given the present conditions, the reason you can't do it is.... If this speaker were referring to me, he would be wrong. The reason I can't sprint a marathon is because I can't even jog a marathon.
"The reason you couldn't sprint a marathon..." - the condition is left unstated, but we can reasonably guess what the speaker meant. Even if you trained your whole life, if you were the fittest and fastest runner in the world, you could not sprint for 26 miles.
You could also express the latter thought using the future tense:
"The reason you will never be able to sprint a marathon..."
