I am on f1 visa and filed for my CPT. While filling the W4 form, unknowningly I had written EXEMPT in Employee Witholding Allowance. After that I got my 3 pay checks with no tax being charged to me. But I am not really sure on what to put in the line no. 7 of w4 form. Is it surely Exempt? Or something else for F1 student who has filed for CPT. Thanks

You are not an exempt employee. File a new W-4 with your employer and claim 0 or 1 allowance.

On F1/OPT/CPT you are exempt from residency for 5-years. You must file non-resident tax return Form 1040NR or 1040NR-EZ and Form 8843. Your income is not subject to FICA taxes but you must pay federal and state income taxes. On your income you will get deduction based on tax treaty between the U.S. and your country. Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents (http://taxipay.blogspot.com/2008/03/us-tax-filing-requirements-for-non.html)

Agreed. Non-resident aliens, like yourself on F1/OPT/CPT , are not permitted to claim "exempt"in the line no. 7 of w4 form . If the person is eligible to claim a tax exemption under an income tax treaty, then he/she needs to complete the Form 8233 in addition to Form W-4. Form W-4 completed will apply at the time a treaty-based exemption no longer applies."