What is the -ē form in “Latīnē loquor”?

Question

To say you speak in Latin or in Greek, you say “Latīne” or “Graecē”. What is that -ē form? I cannot figure it out from any declension table I am aware of.

Does the same -ē ending work for any other language, or is it only a set form for some languages? E.g. can you say “Francogallicē” or “Japonicē”, or do you need to use some standard case & declension (ablative maybe)?

Answer 1

That is the adverbial ending. To convert a first/second adjective to an adverb, you replace the declension ending with -ē: if a document is hard to understand, it's obscūra (or obscūrus etc), but if I do something in a way that's hard to understand, it's obscūrē.

For third adjectives, -iter is more common: a celer person acts celeriter.