Since a modern Hebrew speaker hasn't responded to these questions, I will give it a try. In general, הערב means "this evening" but can also mean "tonight" while הלילה means "tonight" (לילה has base sense of "night"). Source: Giore Etzion, The Routledge Introductory Course in Hebrew, 94. So a person can ask, מה את עושה הערב, "what are you doing tonight?" Since there's not a big difference between "tonight" and "this evening" in English and since the demonstrative pronoun isn't required, it can be confusing for an English speaker. Similarly, היום, "this day," that is, "today." In classical Hebrew, לילה is night while ערב is evening, although in late classical Hebrew ערב could mean night (Job 7:4) and so one can see how in the revival of Hebrew there could be a close connection between the two. לילה is found across Semitic languages. There is a discussion of the morphology of לילה in Pérez Fernández, Introductory Grammar of Rabbinic Hebrew, 5. There's now a grammar of Eastern European Hasidic by L. O. Kahn (Brill, 2014) that can help with a major piece of evidence for the idiomatic use of these words before Israeli Hebrew. In that book, ערב tends to be rendered with "evening" and לילה tends to be "night" but is sometimes "evening." For instance, ליל שבת is "Friday evening" (p. 274). בכל ערב, "every evening" (p. 389). That important book demonstrates how influential eastern Aramaic syntax and vocabulary was because the Hasidim were immersed in the Bavli. In "streetwise Hebrew" podcast # 102, "it's this terrific evening" is זה הערב הזה המשגע: https://www.pastimeapp.com/pod/Streetwise-Hebrew#/
Very nice discussion. I think the point about "tonight" being a possible translation of הערב can be summarized: literally it's not a right translation. In both languages there is nowadays a clear and similar difference between "night"/לילה and "everning"/ערב. But I think (not sure) that when it's not important to be precise, English speakers would tend to prefer "tonight" while Hebrew speakers would tend to prefer הערב. So practically they would often be the best translations of each other.