In the following sentence, why is "can" more appropriate than "could"? Is it a grammar thing? Please explain.
The more satisfied you are with your job, the more effort you could put into your work.
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Both "can" and "could" are ok, but "can" is more natural assuming the sentence is trying to convey a universal truth.
Using "could" would make the sentence feel more hypothetical, or possibly make reference to a past time, while "can" seems to match the present tense of the first clause "you are".
Since the sentence seems intended to assert that something is the case in general, the idea that the ability to make effort depends on satisfaction, then the use of can seems best.
Neither "can" nor "could" is appropriate here. In this context, both refer to ability, and your ability to put effort into your work is not dependent on your level of satisfaction. So we're really talking about probability here, not possibility or ability, which means we want "might":
"The more satisfied you are with your job, the more effort you might put into your work."