So I've been studying sindarin slowly over the last few months. (In other words I'm only at lesson 6 in 5 months). I'm a little confused though on when you are supposed to use formal vs informal speech.
Additionally, and I probably should have asked this sooner, when do you use each type of greeting (galu vs suilad and gin/Len suilon vs g/l'ovannen) ? And how would you say "We are well"?
Let's see if I can help a bit. You'd use formal to anyone in a position of authority (some of this will be personal depending on your own culture and opinions, so I would personally use the formal to a boss at a workplace, not everyone would). Some people would use it for family members. You'd also use it for a stranger. Informal is for anyone you know well, or it could be used if you want to talk down to someone.
I think of Galu as similar to Ciao, it's a quick greeting like 'Hi!', 'Salut!'. Suil/Suilad literally means 'Greetings'. I don't think of it as either formal or informal - it's used by Aragorn in his letter to Sam, but they were friends, so I don't think it's a formal 'King to his subject' style greeting. Mae g'ovannen was used by Glorfindel to Aragorn, long time friends. The formal version Mae l'ovannen, any formal situation really.
Think of English where we can say Hello or Hi, or How do you do; saying Mae g'ovannen or Suilad are just different ways of saying the same thing. I'd say Mae g'ovannen/Galu/Suil/Suilad to my friend, Mae l'ovannen to a stranger or my queen.