If you are a student whose permanent home is Connecticut and you lived in another state temporarily while attending college, you are sill a legal resident of Connecticut. If you are single and your gross income for the year exceeds $12,625, you must file a resident return reporting all of your income for the year.
If your permanent home is elsewhere, but you lived in Connecticut while attending college, you may be a nonresident of Connecticut. If you earned income from a job in Connecticut , you are single, and your gross income for the year exceeds $12,625, you must file Form CT-1040NR/PY.
Remember: Even if your gross income is less than $12,625, you must file a Connecticut income tax return if Connecticut income tax was withheld from your wages or if you made any payments of estimated Connecticut income tax. If you know that your gross income for the entire tax year will be under the filing threshold for your filing status, and you know that you will not meet any of the other filing criteria, enter "E" for "Exempt" as your filing code on Form CT-W4 and give the form to your employer. By doing this, there will be no Connecticut tax withheld from your paycheck and as long as you are under the gross income limit, you will not be required to file a Connecticut income tax return.