In the following sentence, why is "can" 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.
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."