Learn Polish: Wracając do tematu, musimy ustalić termin. - Getting back to the subject, we have to fix a date.

Wracając is an adverbial participle meaning returning or getting back. You create it by taking the third-person plural of an imperfective verb (like wracają) and adding -c. We use this form to show that two actions by the same subject are happening at the same time.

Why do we use the preposition do here, and what case follows it?

The verb wracać (to return) pairs with the preposition do when you are returning to a place or an abstract concept like a topic. The preposition do always requires the genitive case, which is why temat becomes tematu.

Why does tematu end in -u rather than -a in the genitive case?

Temat is an inanimate masculine noun. In the genitive case, these nouns can end in either -a or -u. While there are some patterns, it is often something you just have to memorize. However, abstract concepts and loanwords like temat usually take -u.

Why is the perfective verb ustalić used here instead of its imperfective equivalent ustalać?

We use the perfective ustalić because the goal is to successfully set or fix a date once as a completed action. If we used the imperfective ustalać, it would imply an ongoing process of negotiating or trying to figure the date out.