I never heard of a narthex before. I just learned something, so I guess the day wasn't a total loss.
Narthex = Entry area between outside and the nave. May be open to the nave, may be separated by a curtain/curtain wall, or may have doors between the two areas. The colder the climate the less open between the narthex and nave.
Nave = Area where the congregation is gathered for worship. Where the pews or chairs for attendees are.
Transept and Transept Aisle = In a church building that is shaped like a cross, this is the area that forms the cross beam. Some people will refer to the bump outs as wings. These "wings" are connected by the transept aisle - The aisle that crosses the front of the naive from side to side.
Sanctuary = Historically this was the area, often a curtained room, behind the altar. However, over the centuries it became the area containing the altar. In many churches there will be a rail, low wall, or kneeler with a rail separating the naive from the sanctuary. See my photo and you will see the communion rail that separates the two areas. The designation change came because the areas behind the altar, where only priests entered, was omitted from churches.
Today very few people use the term naive. Most congregations refer to the entire worship area as the sanctuary. I have even noticed that the Catholic Church is making the switch. Also, the term narthex is becoming less and less common. It isn't dependent on architecture, but more churches are utilizing this area for gathering and social activities before and after services. Also, changing away from terms like sanctuary and narthex to gathering space, auditorium, entry, fellowship space, are more inviting to younger generations.