Breakdown of Wir prüfen die Maschine zehnfach, nicht nur zweifach.
wir
we
nicht
not
nur
only
prüfen
to check / to test
Questions & Answers about Wir prüfen die Maschine zehnfach, nicht nur zweifach.
Can I use zehnmal instead of zehnfach in this sentence?
You could, but it changes the nuance slightly. zehnmal means doing the action ten separate times. zehnfach means 'tenfold,' which implies a multi-layered or ten-stage testing process rather than just repeating the exact same test ten times.
Why don't zehnfach and zweifach take adjective endings here?
Because they are functioning as adverbs rather than adjectives in this sentence. They describe how we test (the verb prüfen), not a noun. If they were placed directly before a noun, they would need endings, like in eine zehnfache Prüfung (a tenfold test).
Does nicht nur always require a sondern auch (but also) later in the sentence?
Not always. While nicht nur ... sondern auch is a very common paired conjunction, here nicht nur zweifach acts as a clarifying afterthought. It simply means 'not merely twofold' and stands perfectly well on its own without needing the second half of the conjunction.
Could I say Wir prüfen die Maschine für das Zehnfache to mean 'we test it tenfold'?
No, that would sound like you are testing the machine in exchange for ten times the usual payment! To describe the thoroughness or manner of the test itself, you just use the uninflected adverb form zehnfach.