Learn Hardcore Indonesian: Undanglah guru musik ke pesta agar suasana lebih meriah. - Invite the music teacher to the party so the atmosphere is livelier.

The -lah attached to an imperative verb adds a polite emphasis or exhortation. In English it’s like saying “please invite” or “do invite.” You could simply say undang, but undanglah sounds more persuasive or formal.

Why is there no subject pronoun before undanglah?

In Indonesian imperatives, the subject (you) is usually dropped because it’s understood. Undanglah automatically means “(You) invite…” without needing “kamu” or “Anda.”

What does agar mean, and can I replace it with supaya?

Agar means “so that” or “in order to” and introduces a purpose clause. Supaya is interchangeable in most contexts:
Undanglah guru musik ke pesta supaya suasana lebih meriah.
The nuance is the same—both link the invitation to the desired outcome.

Could I say suasananya lebih meriah instead of suasana lebih meriah?

Yes. Adding -nya makes suasananya “the atmosphere” in a more definite sense. Without -nya, suasana feels general (“atmosphere”). Both are correct:

…agar suasana lebih meriah.
…agar suasananya lebih meriah.

Why is it guru musik and not musik guru?

In Indonesian compound nouns, the head noun comes first and the descriptor second. Guru is the head (“teacher”) and musik describes which kind. Reverse order—musik guru—would be ungrammatical.

Is it okay to omit agar and just use a comma?

You can write
Undanglah guru musik ke pesta, suasana lebih meriah.
but without agar or supaya it becomes two coordinate clauses rather than a clear purpose clause. It’s still understandable in casual speech, but using agar makes the intention (“so that the mood becomes more festive”) explicit.