Learn Polish: W poniedziałek pracuję w domu.

Poniedziałek is a masculine inanimate noun (a thing, not a living person or animal). For these types of nouns, the Accusative form looks exactly the same as the basic dictionary form. So it is in the Accusative case here, but the spelling doesn't change!

Why is the word "w" used twice here for two different English words ("on" and "at")?

The Polish preposition w is very common and translates differently depending on the context. With days of the week, it means "on" (like w poniedziałek). With physical locations, it usually means "in" or "at" (like w domu).

Why doesn't the sentence say "Ja pracuję"?

In Polish, the verb ending already tells us who is doing the action. The -ę ending on pracuję means "I work". You can add ja for emphasis, but it is usually dropped because it is not necessary.

Why does "dom" (home) change to "domu" at the end?

When you talk about being physically located inside a place, the preposition w requires the Locative case. For the noun dom, that form is domu.