Learn Spanish (Latin America): —Buenas tardes, señor. —Buenas tardes.

In Spanish, a long dialogue dash is used to show that someone is speaking. It takes the place of English quotation marks and appears at the very beginning of the speaker's turn.

Why is it buenas tardes here, but we say buenos días in the morning?

Greetings must match the gender of the noun. The word for afternoon, tarde, is feminine, so we use the feminine plural form buenas. By contrast, the word for day, día, is masculine and therefore takes buenos.

If I continue speaking to this señor, should I use tú or usted?

You should use usted. Addressing someone as señor (sir) shows respect and formality, which is exactly when the formal usted is required. If you were close enough to use the informal tú, you would likely just call them by their first name.

At what time of day do people start saying Buenas tardes?

In Latin America, people switch from Buenos días to Buenas tardes right at midday (noon). You continue using it until it gets dark out, which is when you switch to Buenas noches.