Alphabet Soup - Visa's

"A" Visa - diplomats, foreign government officials - days are exempt unless green card holder
"G" Visa International organization employee - days are exempt unless green card holder.
"J" Visa teacher or trainer -Days are not exempt and must be included if the teacher/trainer was a student, teacher or trainer in any two years in the past 6 calendar years.

If compensation paid by foreign employer then as long they were present as a teacher or trainer in any prior year in the past 6 years and during that period was only paid by a foreign employer for teaching/training.   The individual was exempt for 3 or fewer years of the previous 6 calendar years as a teacher, trainer, student. 

Student Visas: 
A student includes any individual ( and that individual's immediate family), who is temprarily admitted to the US:
- Under "F" or "M" visa (relating to students) or "J" (teacher or trainer) or "Q" cultural exchanges.  
A student is not exempt and must include all of the days presence as a student, if the individual has been exempt as a student, teacher, trainer in the past 5 calendar years.  However, this limitation will not apply if:  
 establishes no intention of permanent establishing residence in the US and complies with the Visa requirements.

​Professional Athletes:
Days are only exempt if the event's main purpose is to benefit a charitable organization, the entire proceeds go to charity & volunteers perform substantially all of the work.  Only the days that the athlete actually competed are excluded. 

Any days exempted due to the above get reported on Form 8843 which has to be timely filed to get the days exempted.  Even if you don't have to file a tax return, still file Form 8843 to get the days registered as exempt.