Learn Hardcore Dutch: Volgens het rooster begint het vak wiskunde om acht uur. - According to the timetable, the subject Mathematics starts at eight o’clock.

volgens is a preposition meaning “according to.” It introduces the source of information or authority. You always follow it with a noun or noun phrase (often with an article):

volgens het rooster (“according to the schedule”)
volgens mij (“in my opinion,” literally “according to me”)

Why is the verb begint placed immediately after volgens het rooster?

Dutch follows the “V2” (verb-second) rule in main clauses: whichever phrase (subject, time, adverbial) you put in first position, the finite verb must come second. Here the first position is occupied by volgens het rooster, so begint (the finite verb) comes next.

What does rooster mean here, and is it interchangeable with schema?

In a school context, rooster means “timetable” or “schedule” (the plan of lessons). schema can also mean “schedule,” but it’s more general (e.g. a work schedule, project chart) and less common for school timetables.

Why is it het vak wiskunde instead of just wiskunde?

vak means “subject” (as in school subjects). Saying het vak wiskunde emphasizes “the subject mathematics.” If you simply said wiskunde, a listener would still understand, but you lose the explicit “subject” nuance. You could also say de wiskundeles (“the math lesson”) if you mean the class session.

Why do we use om before acht uur, and can it be omitted?

To specify a clock time in Dutch, you normally introduce it with om (“at”):

om acht uur (“at eight o’clock”)
You wouldn’t drop it in a full sentence. In short answers or headlines you might see just acht uur, but in standard declarative sentences you keep om.

Could I start the sentence with om acht uur instead of volgens het rooster?

Yes. You could say

Om acht uur begint het vak wiskunde volgens het rooster.
The first position is now om acht uur, the finite verb begint remains second, and the rest follows. The meaning stays the same, though the emphasis shifts slightly to the time.

Is beginnen the only verb for “to start,” or can I use starten?

Both are used. beginnen is more general and somewhat more common, especially in school contexts. starten is a loanword from English and might sound a bit more formal or technical (e.g. de motor starten, “to start the engine”). You could say Volgens het rooster start het vak wiskunde om acht uur, but begint is idiomatic here.