as hard as you can vs as hard as you could

Question

(1) You are not working as hard as you can to learn how to ski.

I think there is nothing with (1). I will change the modal verb, can, to its past tense.

(2) You are not working as hard as you could to learn how to ski.

Besides the tense difference, how does the past tense, could, change the meaning of the first sentence in which "can" is used?

Answer 1

'Could' exists both as a modal (auxiliary) verb and as the past tense of the modal verb 'can'.

This (with some reformating for clarity) is from the M-WD definition of 'could'.

could auxiliary verb

1. past tense of CAN

-- used in auxiliary function in the past e.g. We found we could go.

-- used in the past conditional e.g. We said we would go if we could.

2. an alternative to CAN, suggesting less force or certainty or as a polite form in the present e.g. If you could come, we would be pleased.

In your example, 'could' is being used meaning #2, as a less forceful alternative to 'can'.