'Unless' means 'if not' and creates a conditional statement. The sentence 'Unless Sohan finishes this exercise he will not read the novel' means Sohan will not read the novel if he does NOT finish the exercise - that is, finishing the exercise is a necessary condition for reading the novel. 'While' and 'because' don't fit this conditional meaning, 'when' is for time reference, and 'even if' suggests a contrary condition.