This confused me so much, initially. I kept read it as already time or already one time and it just got nowhere. I eventually started seeing schon (ein)mal as been and with warst as have been. This is absolutely something you need to learn as I see and hear this usage a lot. It’s a lot like our have you ever been, I think.
If someone asks you, “warst du schonmal in Deutschland,” then they’re asking whether you have ever been in Germany, so what they’re essentially saying, with broken English as an example, is, “were you already one time in Germany?” I know, I said I stopped thinking of it as already one time, but it’s actually a good way of looking at it, in my opinion, but also to strongly keep in mind the to be side of it, and it’s this state of being that helped me understand it.
This usage also works with things as well as to have been to places or to have done something verby, such as swimming, running, jogging, and so on and so forth. If someone asks you, “warst du schonmal Krank,” they are asking whether you have ever been sick; whether you were, at one point, sick. “Warst du schonmal joggen?” Have you ever been jogging?
We do have this usage in English, it’s just not used often. One such example could be, “were you ever aware (at some/any point) that [something],” which works quite well in English, and sounds quite proper. I’m sure there’s a proper term for this, but for the life of me I cannot figure that out as I’m far better with German terminology [die Terminologie; pl. die Terminologien], but it’s simply referring to the past, going by the basic conjugation of you were, vs the likes of it was.
Schon mal can be written as schonmal, schon mal, or schon einmal, although I have my suspicions that grammar freaks might insist on separating the two words. I believe, but could be entirely wrong here, that schon einmal adds emphasis to the “one time” aspect of it, possibly akin to our “at some point” or “at any point.”
I wasn’t even altogether sure of how you would express this with to be as I simply never usually see it, but after checking up on my guess, it appears I was bang on the money, in that you would say something like, “bist du jemals in Deutschland gewesen?” The jemals seems to take the place of einmal and is probably a closer translation of at any point or our simpler ever. Remember, gewesen is a sein word, hence it is bist and not hast, in this example.
There is another usage for schonmal that you should heed, as have I just discovered. Here’s an example I found on Linguee, “bist du schon mal ins Ausland gereist?” Which essentially has the same sort of meaning as the usage explained above, but this is adding emphasis to traveling to somewhere and is asking whether you have gone into a country (to go abroad) rather than simply whether you have been in one—a small difference, perhaps, but a useful one to know, none-the-less. I should point out that I don’t see schon here as been, rather, I see it as already.
I hope that was useful, and I’ll be sure to make any necessary corrections should mistakes [der Fehler; pl. die Fehler] arise. If you found this helpful, please let me know in the comments below, and if you have any tips for how to remember these usages, or if you have any other usages that you know of, then please let us know!
Random tip: it’s colloquial to call a comment online, “(der) Kommi,” but keep in mind that it can also mean communist!