¡Buenas tardes!

Because tardes is a feminine plural noun, so the adjective has to agree with it.

• bueno = masculine singular
• buena = feminine singular
• buenos = masculine plural
• buenas = feminine plural

Since tardes is feminine plural, you say buenas tardes.

This is just how Spanish expresses the greeting. Spanish uses:

• buenos días
• buenas tardes
• buenas noches

So even though English uses the singular in good afternoon, Spanish uses the plural form tardes. It is something you should learn as a fixed expression.

In Spain, people very commonly shorten it to Buenas, especially in casual situations.

For example:

• entering a shop
• greeting neighbors
• greeting coworkers informally

Buenas tardes sounds more complete and a bit more neutral or polite.
Buenas is normal and natural, but slightly more informal.

In Spain, buenas tardes is generally used from after midday until evening.

A rough guide:

• buenos días: morning, often until lunch time
• buenas tardes: afternoon and early evening
• buenas noches: night, or when it is clearly late / dark, or when saying good night

In Spain, meal times and daily schedules are often later than in some English-speaking countries, so buenas tardes may be used later than you expect.

Spanish uses two exclamation marks:

• ¡ at the beginning
• ! at the end

So ¡Buenas tardes! is the correct full punctuation.

In informal texting, some native speakers may leave off the opening mark, but in correct writing you should include both.

It works in both formal and informal situations. It is a standard greeting.

You can say it to:

• friends
• strangers
• shop assistants
• teachers
• coworkers
• older people

What changes the level of formality more is what comes after it, for example:

• Buenas tardes, señora
• Buenas tardes, ¿qué tal?
• Buenas

So the phrase itself is very safe and widely usable.

No. Buenas tardes is a complete greeting by itself.

It is not a full sentence in the literal grammar sense, but it functions as a fixed expression, just like many greetings do. You do not say something like Yo buenas tardes.

You can simply say:

• ¡Buenas tardes!

And that is perfectly natural.

A simple pronunciation guide is:

BWEH-nas TAR-des

A bit more closely:

• bue- sounds like bweh
• -nas has a clear s
• tar- has a tapped Spanish r
• -des ends with a soft s

In much of Spain, the d in tardes is softer than English d, especially between vowels. You do not need to exaggerate it.

Normally, only if it begins a sentence or stands alone as the greeting line.

So these are correct:

• ¡Buenas tardes!
• Le dije buenas tardes al entrar.

In the second example, buenas tardes is not capitalized because it is in the middle of the sentence.

Yes. In practice, it functions as a greeting, so depending on context it can correspond to:

• good afternoon
• hello / hi (used in the afternoon)

The exact English translation depends on how it is being used, but in Spanish it is one very common greeting formula.

Both are standard greetings, but they are used at different times.

• Buenas tardes = afternoon / early evening greeting
• Buenas noches = night greeting, and also good night

Unlike English, Spanish buenas noches can be both a greeting and a farewell.
Buenas tardes is mainly a greeting.

Usually, no. Buenas tardes is mainly used to greet someone, not to say goodbye.

When leaving, Spanish more often uses:

• Adiós
• Hasta luego
• Que tenga buena tarde or que pases buena tarde in some regions/contexts

So if you are entering a place, ¡Buenas tardes! is perfect. If you are leaving, another expression is usually better.