Learn Hardcore Swedish: Du är Erik.

Yes, du is generally used in all contexts nowadays. Historically, Ni (you, plural/formal) was sometimes used in more formal situations, but modern Swedish usually sticks to du, even in polite settings.

Does är change depending on the subject, like "am," "are," or "is" in English?

No, är stays the same no matter what the subject is. In Swedish, there's only one present-tense form of the verb "to be," which is är for jag (I), du (you), han/hon/den/det (he/she/it), vi (we), ni (you plural), or de (they).

How do I pronounce är correctly with the Swedish vowel ä?

The ä sound in är is somewhat similar to the "a" in English words like "care" or "share," but a bit more forward in your mouth. It might help to relax your jaw slightly and keep your lips unrounded when saying ä.

Is there any special rule for capitalizing du or names like Erik in Swedish?

Names like Erik are always capitalized, just like in English. However, the pronoun du is not capitalized unless it comes at the beginning of a sentence. In Swedish, we generally don't capitalize pronouns (like I in English), so du remains lowercase unless it starts a sentence.