Learn Polish: Odpoczywam, bo dużo pracuję. - I am resting because I work a lot.

Yes! In Polish, you must always put a comma before subordinating conjunctions like bo (because) and że (that). Even though in English we do not usually put a comma before the word 'because', in Polish it is a strict punctuation rule you need to follow.

Why doesn't the sentence say Ja odpoczywam and ja pracuję?

In Polish, the subject pronoun ja (I) is usually dropped because the endings of the verbs already tell you exactly who is doing the action. Both odpoczywam and pracuję have endings that specifically mean 'I' am doing it. Adding ja would sound unnatural here unless you wanted to strongly emphasize that you are the one resting.

If both verbs are in the 'I' form, why does odpoczywam end in -am while pracuję ends in -ę?

Polish verbs fall into different conjugation groups based on their endings. The dictionary form of 'to rest' is odpoczywać, which belongs to a group where the 'I' form ends in -am. The dictionary form of 'to work' is pracować. Verbs that end in -ować belong to a special group that always changes to -uję for the 'I' form.

Can this sentence also mean 'I rest because I work a lot' instead of 'I am resting'?