Learn Croatian: Kao prvo, izolacija kod kuće ponekad otežava komunikaciju. - Firstly, isolation at home sometimes makes communication difficult.

To say Secondly, you use kao drugo. To introduce a contrast, which fits your current lesson on discourse markers, you would use s druge strane (on the other hand). These phrases are incredibly useful for structuring your arguments!

The verb otežava means makes difficult. How is this verb formed?

It is built from the adjective težak, meaning heavy or difficult. The prefix o- and suffix -avati turn it into the imperfective verb otežavati (to make difficult). If you need the perfective version for a completed action, it is otežati.

Why does the sentence use kod kuće instead of u kući?

The phrase kod kuće is a fixed expression meaning at home in a general, everyday sense. If you were to say u kući, you would specifically be talking about being physically inside the house as a building.

Could I just use Prvo instead of Kao prvo?

Yes, they are completely interchangeable here. Kao prvo literally means as the first thing and just adds a tiny bit more structure when you are about to list multiple arguments or points.