Look at the tax return when it is marked as Single, then look at the tax return again when it is Married Filing Separately. Look at which lines change, and you have your answer. If you are unsure why a specific line changes and want help understanding that, come back and tell us what line change you do not understand.
Many deductions and credits are not allowed for filing as Married Filing Separately.
Out of curiosity, why are you comparing the two? It is not an option to pick between the two.