What is the difference between 依然 and 仍然?
依然 and 仍然 both can be used as adverb to mean "still". And when used this way, they are interchangable in most cases. You can say "虽然生病，他依然去上学了" or "虽然生病，他仍然去上学了". Both are appropriate.

However 依然 can also be used as a verb, such as "noun + 依然" (most likely in some idioms or phrases). e.g 旧态依然, 风采依然. You cannot use 仍然 in these situations. But anyway, such kind of usage is rarely heard in daily talks.

In addition you'll find 依然 in some classic poems. But I haven't seen any occurence of 仍然 in poetry before 20th century.