Talking and Flirting with the opposite gender:
It is permitted for a man to talk to a woman regarding work or business related subjects [serious and to the point conversations]. It is however absolutely forbidden to flirt with a girl/woman or do any lightheaded activity with her. Doing so transgresses a Biblical negative command [of Lo Sikravu]. It goes without saying that one may not say to her words of affection that can lead towards feelings for one another, even when talking of business related subjects, as even the saying of Shalom to a woman is forbidden for this reason, as explained next. It is certainly forbidden to talk with her regarding intimate subjects that can lead to Erva.
Saying Hello/good morning: See next!
May a man be a woman’s therapist?
May a male/female tutor a person of the opposite gender?
 See Michaber 21/1 and Ohel Yaakov p. 169
 See Beis Shmuel 21/4 “However hearing her voice during speech is permitted”; There is no prohibition found in Poskim regarding interaction with a woman for business related matters or other everyday activity, and so is the custom even amongst G-d fearing Jews to have work conversations with women when necessary.
 See Michaber 21/1 “It is forbidden to have fun with her [i.e. Lischok Ima], to be light headed with her.”; Based on this Rama Y.D. 195/1 explicitly prohibits flirtatious conversation regarding a wife who is a Nida; If even asking Shalom to a woman is forbidden [as brought in Halacha ??] then certainly flirting with her is forbidden Upashut! Those that state that the Shulchan Aruch does not forbid flirting are walking in darkness and miss the entire spirit of the laws mentioned here by the Michaber, aside for the fact that even saying Shalom was explicitly prohibited. There is no greater gateway action that leads towards promiscuity than flirting with the opposite gender. It is certainly included in the ruling of Michaber 21/1 “One must distance himself very very much from women”
 Michaber ibid “All the above matters receive Makas Mardus and the above matters are forbidden also due to a negative command”
 See Rama Y.D. 195/1