Learn Swedish: Jag vet inte varför datorn inte startar. - I do not know why the computer does not start.

This is a perfect example of the BIFF rule! The first half, Jag vet inte, is a main clause, so inte goes after the verb. The second half, varför datorn inte startar, is a subordinate clause. The BIFF rule reminds us: in a Bi-sats (subordinate clause), Inte comes Före (before) the Finita verbet (verb).

Does the word varför always make the sentence a subordinate clause?

When it connects two parts of a sentence like this, yes. Here, varför creates an indirect question. When question words like varför (why), när (when), or vad (what) are used in the middle of a sentence, they act as conjunctions and trigger the BIFF rule word order for the phrase that follows.

I learned that en-words get an -en at the end for the definite form. Why is it datorn instead of datoren?

While most en-words take -en, nouns that end in -or are a common exception. Words like en dator (a computer) or en doktor (a doctor) simply take an -n in the definite singular form, becoming datorn and doktorn.

Could I use känner instead of vet here since they both mean "know"?