Questions & Answers about Peter ist sehr fleißig, Tom ist dagegen extrem faul.
Why is dagegen in the middle of the second clause instead of at the beginning?
dagegen is an adverb meaning "on the other hand" or "in contrast." Unlike coordinating conjunctions like aber (but), which usually sit right between clauses, adverbs are flexible in a sentence. Here it is in position 3, but you could also put it in position 1 and swap the verb and subject: Dagegen ist Tom extrem faul.
Could I just use aber instead of dagegen in this sentence?
Yes, you could say ..., aber Tom ist extrem faul. However, dagegen strongly emphasizes the contrast between the two people. It translates nicely to "on the other hand," making your German sound more varied and precise than just relying on aber.
Why is there just a comma separating these two complete sentences?
In English, joining two independent clauses with just a comma is usually considered an error known as a comma splice. In German, it is perfectly grammatically correct and very common to link two closely related main clauses with just a comma, especially when showing a direct contrast like this.
Are fleißig and faul the standard words for this kind of contrast?
Yes, fleißig (hardworking, diligent) and faul (lazy) are the classic opposites to describe someone's work ethic in German. As a bonus tip: faul can also mean "rotten" when talking about food, such as ein fauler Apfel (a rotten apple).