Curricular Practical Training (CPT) is an employment opportunity in the United States available to an international student in their field of study while in valid F-1 visa status. 
CPT experience can be paid or unpaid. A job offer letter is required at the time of application.
Your application must be submitted at least two weeks before employment begins
You must be enrolled full-time at a service-approved college, university, or seminary for at least 1 academic year prior to beginning CPT
CPT start date must be on or after the date that CPT is authorized in SEVIS. Students cannot legally work before CPT is authorized in SEVIS.

In order to be considered CPT, the experience must be a “required” or “integral part of the established curriculum” and “directly related to the field of study.”
Employment which is just in your field of study does not qualify for CPT! It must be an established part of your academic curriculum. CPT can only be done before completion of your degree program. 
Graduate students who miss the graduate school’s graduation deadline are considered to have completed their program and are no longer eligible for CPT the following semester (even though you are required by the graduate school to register for Continuous Registration).

Part-Time CPT (20 hours per week or less) - Will not affect eligibility for OPT. 

Full-Time CPT (More than 20 hours per week) - Students who use 12 months of full-time of CPT will lose eligibility for OPT.

OPT is temporary employment for practical training in your field of study (the major/program listed on your I-20) 
OPT is a benefit of your F-1 status. It is not a separate status. OPT is available at each progressing degree level. 
OPT can be approved for up to 12 months of employment authorization. OPT requires approval from USCIS (United States Citizenship and Immigration Services).
Can be done either before (pre-completion) or after (post-completion) your degree program.

Used for employment before your academic program is completed.  
Limited to 20 hours per week during fall and spring semesters. Does not qualify for STEM extension. 
May extend I-20 if extra time is needed. Pre-Completion OPT time is subtracted from Post-Completion eligibility

Used for employment after an academic program is completed. 
Unemployment counts (limited to 90 days). 
Must work full-time (20 hours per week or more). 
Eligible for H-1B cap gap extension. 
Cannot extend I-20 if degree is not completed.

Before applying for post-completion OPT, you must determine your program completion date. 
Fall Grad - Dec 15, 2025
Spring Grad - May 8, 2026
If you need to extend your program because you have not fulfilled your requirements, you must make that request before the program end date. 

You can apply for OPT up to 90 days before and up to 60 days after your program completion date. 
You must choose your requested OPT start date. 
This can be as early as the day after your program completion date or as late as 60 days after your program completion date.
Link:https://studyinthestates.dhs.gov/sevis-help-hub/student-records/fm-stud…