Your question has already been answered, but let's make the point more generally: A language learner's perspective on a language will be completely different from a native speaker's. The learner's attention will always be focused on grammatical properties of lexemes: separable or non-separable verb, strong or weak declension, strong or weak preterite, etc.
Wann fangen die Ferien an? – Wann beginnen die Ferien?
der Insasse, den Insassen – der Häftling, den Häftling-∅
tragen, trug – schleppen, schleppte
They need to do so in order to pick up the grammar (unless they are children and can learn by immersion alone).
But native speakers are usually completely oblivious to the grammatical properties of lexemes, unless they have been taught to pay attention. (And paying attention to the structure of language can become a hobby, which is when you join Stack Exchange…) It follows that usage is independent of a lexeme's grammatical properties: Non-separable verbs are not used in other contexts than separable ones.
By grammatical properties I in this case specifically mean oppositions between lexemes, as illustrated by the examples, and not oppositions between word forms or grammatical categories such as trägt (present) vs. trug (past).