While practicing writing this sentence fragment (“First you must select a good cookbook, then ...”) in German using each of my two ‘silent tutors’(translate.yandex.ru and translate.google.com) to check the correct way to say or write that fragment I found that the translations returned by my two tutors was functionally the same, but that they had differences in word order and familiarity with the person to whom my sentence fragment might have been directed.
Here is what my two tutors had replied:
(Yandex): Zeurst müssen Sie wählen Sie ein gutes Kochbuch, dann ...
(Google): Zeurst musst du ein gutes Kochbuch auswählen und dann ...
In checking my German, my Russian tutor (Yandex) returned my sentence in ‘polite’ word usage, while my American tutor (Google) used ‘familiar’ word usage. However, you’ll notice that both of my tutors went on to consistently correct my use of the word “Erst” (an adjective) to “Zuerst” (an adverb) whenever I made small revisions to either the English or the German sentences.
(A grammar anomaly indirectly related to my question is in the American version (Google) where “und” is required before the word “dann” -- why, I don’t understand.
My question is this: Since in the ‘Ordinal Numbers’ section in many of my German grammar books (including Duden’s “Die Grammatik”) the word “zuerst” is not mentioned nor defined; the only German grammar book that did mention it was only in the “Wörterverzeichnis” section of my English language grammar book Colloquial German (by P. F. Doring, 1975 edition or later).
So, is the word “zuerst” okay to be used, but okay only in colloquial [i.e., everyday] German speech and writing, and is the “zu” in that word a separable prefix?