The English translation ("Do you know Lema") is a bit confusing as it might give the impression that Lema is the object of the verb. Of course the actual meaning is: "Do you know, Lema?" Or "Lema, do you know?"
Yes, there is a coma in fact before "Lama" in the English sentence above.
In Arabic, however, there is the vocative article (Yá) before the name of the person we are talking to. So, we know that the person is asking a person called Lamá and not asking about knowing Lamá.
What is the difference between هَل تَعْرِفين and هَل تَعْرِف ? I guess it is a question of male / female here. This lesson really deserves a explanation. So many critical pieces of information.
Yep. تعرف is a speech dedicated to male and تعرفين to a female (regardless of the gender of the speaker him or herself - have to state this as some people thought there is such a thing).
Anta ta3rif أنتَ تعرف = you know (speaking to a male).
Anti ta3rifeen أنتِ تعرفين = you know (speaking to a female).
Antum ta3rifún أنتُم تعرفون = you know (speaking to group of males or mixed genders).
Antunna ta3rifn أنتُنّ تعرفن = you know (speaking to a group of females only).
Antumá ta3rifán أنتُما تعرفان = you two know (speaking to two persons/Dual, regardless of gender).