Learn Swedish: Vi bekräftar att polisen är här. - We confirm that the police are here.

Att translates to "that" and is used here to introduce a subordinate clause (a secondary part of the sentence that provides more information). It connects the main action (Vi bekräftar) with the fact being stated (att polisen är här).

Can I leave out att and just say "Vi bekräftar polisen är här", like we drop "that" in English?

Yes, in everyday spoken Swedish, people often drop att just like we often drop "that" in English. However, it is grammatically safer to keep it, especially in writing, to make the structure of your sentence perfectly clear.

Why does the English translation say "the police are", but the Swedish uses the singular polisen?

In English, "police" is treated as a plural group. In Swedish, polisen literally means "the police force" (or it can refer to a single police officer) and is a singular en word. Because it is singular, the Swedish thought process is more like "the police force is here."

What is the word order after att?

In a subordinate clause (bisats), the subject almost always comes immediately after the conjunction. So we see att + subject (polisen) + verb (är). If you wanted to make this negative, the word inte would have to squeeze in before the verb: att polisen inte är här.