In English, we wish someone a good day because we expect to see them again, by custom. In Spanish, you wish that all their days or nights be good because you never know when you'll see them again. This is a rough explanation. It's a cultural difference.
When I moved the cursor to "tardes" it translated as "afternoons" (with 's' on the end).
I wrote it as "afternoon" - without the "s" - because it felt right. Fortunately it accepted my answer.
Is there a reason why it is translated as "afternoons" with "s" when I moved my cursor over it?
It's just the way it works in Spanish. The greetings use the plural. I'm not exactly sure why, I'm sure a native speaker on here can tell you more.
Other examples: "¡Buenos diás!" for good day and "¡Buenas noches!" for good night.
first, know Spanish comes from latin like French and unlike (partially) english or german. Consonants are not 'spitted' strongly so the Spanish 't' is different from the English 't', much softer. In fact, the 'd' is very softly pronounced, and sometimes omitted (speaking), specially if the word ends in 'd' or 'ado' (sounds /ao/). Verdad is heard /verdá/ and lado is heard /lao/, though we understand it is not the correct pronuntiation.
"tardes" does mean afternoons. We just do not say "Good afternoons" in English. Well it is 870 times less common in google searches anyway.
I can only ever remember the word 'tardes' because it sounds like TARDIS from Doctor Who...
Why there is a "¡" before "¡Buenas tardes" but not before "Buenas noches"?
I assume Spanish is the same as English. You can use exclamation marks anywhere you want. It just indicates something with more intense feeling.
Yep, there are times when it is mandatory, such as interjections, but you are right.