Learn Swedish: Trots att felet är enkelt kommer han inte att återkomma idag. - Even though the error is simple, he will not get back today.

This is the classic V2 (verb-second) rule in action. The entire first phrase, Trots att felet är enkelt (Even though the error is simple), acts as the first block of information. Because the verb must always take the second position in a main clause, kommer comes right after it, pushing the subject han to the third position.

Since the current lesson is about "inom" (within), why can't we say "inom idag" to mean "within today"?

In Swedish, inom is used with a measurable length or amount of time, like inom två dagar (within two days) or inom en timme (within an hour). You cannot use it with specific calendar days like idag or imorgon. To express doing something before the day is over, you just say idag or use a phrase like under dagen (during the day).

What is the difference between "trots" and "trots att"?

Trots means "despite" and is used directly before a noun, like trots felet (despite the error). However, when you want to use it before a full clause with its own subject and verb (like felet är enkelt), you must add att to make it trots att (even though / despite the fact that).

Why does "enkelt" have a "-t" at the end here?