The Chassidishe Gatesheader writes:
I do get a bit annoyed at “American Yiddish” which is totally strange, such as the above “farshteist” – it’s “farshteit”, without the last s (original German: “versteht” – the only difference is instead of “versteht” which is pronounced, for Americans, as “vershtate”, we pronounce it as “vershteit”).
In German we ask “verstehst du?”, so the Yiddish version “farshteist?” (with s) is not strange at all.
(My mother tongue is Yiddish, and I’m fluent in German too)