"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.
The Ersatz Elevator was created in 2001-03.
An ersatz is something which is made in imitation, as an effigy or substitute for the original.
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.