"Andrew Sarangan" wrote in message A flight instructor certificate is required whenever the FAR calls for
'authorized instructor'. Examples are endorsing a student for solo flight,
giving checkride endorsement, flight reviews etc.. My understanding is that
any commercial pilot can give flight instruction and charge for his
services as long as he does sign students off.

Nobody said anything about charging, so really a commercial isn't even an
issue here. Further, the FAA doesn't hold that flight instruction is even a
commercial piloting operation (an instructor doesn't need a medical if not
needed as a pilot crewmember otherwise), etc...

You can't log flight instruction other than by an authorized instructor. (61.51(b)(2)(v)).
While technically anybody can "teach" someone (no need for an instructor certificate),
it doesn't count as instruction time (for certificates, ratings, BFRs, and other
experience requirements).