"Erstwhile" as an ajective is "former" or "previous." As an adverb, it is "formerly." "Ersatz" is an imitation, generally inferior. A mnemonic device is the "...while" -- related to time, hence former.
In simplest terms Ersatz means Fake or a Substitute.
Erstwhile means former, previous, or in the past.
Erstwhile Records was created in 1999.
An ersatz swing would be an imitated; or fake swing.
An ersatz is something which is made in imitation, as an effigy or substitute for the original.
The Ersatz Elevator was created in 2001-03.
When Joe brought home an ersatz diamond ring, his fiancee threw him out.
Ersatz GB was created on 2011-11-14.
Depending on context, Ersatz can be translated as:surrogatereplacementsubstitutesparealternativerefundcompensationreimbursement
the man was very erstwhile when he walked
Erstwhile Susan was created on 1919-11-16.
After the deaths of his erstwhile cellmates, the surviving prisoner was finally released. I found that my erstwhile partner had been embezzling from our business.