Students, who study hard, should do well on the test.
There should be either two commas or no commas in this sentence.
If only students who study will do well, then use no commas.
If all students study and do well, then use two commas.
=)
To determine if two commas or no commas should be used in the sentence "Students who study hard should do well on the test," we need to consider the logic and meaning of the sentence.
If the meaning is that only the students who study hard will do well on the test, then no commas should be used. This implies that only those students who put in the effort to study hard will achieve good results.
On the other hand, if the meaning is that all students who study hard will definitely do well on the test, then two commas should be used. The two commas would indicate that the phrase "who study hard" is providing additional information about the students, but it is not essential to the meaning of the sentence. In this case, it suggests that studying hard is a common characteristic among the students who are expected to do well on the test.
Ultimately, the decision of whether to use two commas or no commas depends on the intended meaning and the relationship between studying hard and doing well on the test.