Mae govannen! - Sindarin: Well met!
Mára omentië! - Quenya: Well met! (Translated from above, used when two people meet, dual form)
Mára yomentië! - Quenya: Well met! (When there are more than two.)
Suilad! - Sindarin: Greetings! (Formal)
Suilad mae govannen! - Sindarin: Greetings (and) well met!
Aiye! - Quenya: Hail! (Extremely formal, usually followed by title, as in following.)
Aiye Nolofinwë, aran Noldoron! - Quenya: Hail Fingolfin, king of (the) Noldor.
EDIT: Right, there was a typo. But, in this I cannot help quite much on the etymologies. Quenya-wise I'm still a script kiddie, and just based this off Quetin i lambë eldaiva, English edition v1.05, pg. 42. The course, compiled by Thorsten Rank, is available in the Gwaith-i-Pherdain site, http://www.elvish.org/gwaith
For the grammatical difference, it should be a difference between dual and plural, as the Tolkien's elven languages do make distincion between one, two and many, as in some languages still spoken.