According to my 1933 Oxford Universal Dictionary, “good-bye” and “co-operate” are hyphenated, neither “leg room” nor “birth rate” can be run together into single word, and “teenager” doesn’t exist.

Let's talk about the plurals of compound nouns. I submit to you: passers-by, hangers-on, attorneys general, brothers-in-law, and culs-de-sac. What about "month end" how would that be pluralized?

'False friends' are words in one language that look like words in another language, but which have very different meanings. There are a number of false friends in German, where the words look ...