Premises:
You are studying very hard.
You are putting in 10 hrs a day.
You work your way through many questions every day.

Conclusion:
You will pass the test.

Assumption:
Is there a premise you NEED to make the conclusion hold?
"Hard work is sufficient to pass the test"

This is an assumption the author is making. He hasn't said this as such in his argument but he is assuming it. He is assuming that hard work is enough to pass.

The argument would be more complete if it looked like this:
You are studying very hard. You are putting in 10 hrs a day. You work your way through many questions every day. Hard work is sufficient to pass the test. Hence, you will pass the test.

What if I negate this assumption and make it: "Hard work is not sufficient to pass the test"
Can my conclusion still hold?
So you are studying very hard but hard work alone may not be enough. Can I say that you will pass the test? No. This is the point of negating the assumption. If the assumption is negated, the conclusion cannot hold. It is necessary that the assumption must hold if the conclusion has to hold.

If you negate an option and there is still a possibility that the conclusion can hold, it means the option is not an assumption.