La capital recibe un millón de turistas cada año.

Because la capital means the capital. In Spanish, you normally need an article with a singular countable noun like this.
So:

• la capital = the capital
• capital by itself would sound incomplete here

Also, capital is a feminine noun, so it takes la, not el.

In Spanish, nouns have grammatical gender, and capital (meaning capital city) is feminine: la capital.

This does not mean the city is literally feminine; it is just the gender of the noun. Unfortunately, gender often has to be learned word by word.

Examples:

• la capital
• una capital importante

Here it means capital city. Spanish capital can refer to capital city or capital/money/resources, just like English can have different meanings depending on context.

In La capital recibe un millón de turistas cada año, the context clearly points to a city, because cities receive tourists.

Because capital is a common noun here, not a proper name.

So:

• la capital = common noun, not capitalized
• Madrid = proper name, capitalized

The only capital letter in La capital recibe... is La because it starts the sentence.

Recibe is from the verb recibir, meaning to receive.
Here it is in the present tense, third person singular:

• yo recibo = I receive
• tú recibes = you receive
• él/ella recibe = he/she receives

Since la capital is singular, the verb is recibe.

In this sentence, the present tense expresses a general fact or repeated situation: the capital receives that number of tourists every year.

Because the subject is la capital, which is singular.

The structure is:

• La capital = subject
• recibe = verb
• un millón de turistas = object

Even though turistas is plural, that does not control the verb. The verb agrees with the subject, not the object.

This is about the personal a. In Spanish, when the direct object is a person or people, you often use a:

• Veo a María
• Conozco a mis amigos

But with a non-specific or general group, Spanish often leaves it out. In this sentence, un millón de turistas is understood as a general quantity, so recibe un millón de turistas is the normal neutral phrasing.

You may sometimes see a with people in other contexts, but here omitting it is very natural.

Because millón is a noun, and in Spanish numbers like un millón, dos millones, un montón, la mayoría, etc. are followed by de before another noun.

So:

• un millón de turistas
• dos millones de habitantes
• un montón de problemas

You cannot say un millón turistas.

Because the amount is one million, so millón stays singular:

• un millón de turistas

But the thing being counted, turistas, is plural because there is more than one tourist.

Compare:

• un turista
• dos turistas
• un millón de turistas

If the number were more than one million, then millón would also become plural:

• dos millones de turistas

Because cada is followed by a singular noun in Spanish.

So:

• cada año = each year / every year
• cada día = every day
• cada semana = every week

Not:

• cada años
• cada días

This is different from English, where we often use a plural idea in translation: every year.

Yes. Spanish word order is fairly flexible, especially with time expressions.

These are both natural:

• La capital recibe un millón de turistas cada año.
• Cada año, la capital recibe un millón de turistas.

The meaning is basically the same. Putting cada año first gives a little more emphasis to the time expression.

Yes, depending on the context and how natural you want the English to sound.

Literal meaning:

• recibe = receives

Possible natural translations in English:

• The capital receives one million tourists every year.
• The capital gets one million tourists every year.
• The capital welcomes one million tourists every year.

However, welcomes adds a slightly more positive, promotional tone. Receives is the most direct match to the Spanish.

A broad Spain pronunciation would sound roughly like:

la kapi-TAL re-THEE-be un mi-YON de tu-REES-tas KA-da AN-yo

A few useful points:

• c before i in recibe is pronounced like English th in think in most of Spain
• b/v in recibe is a soft Spanish b sound
• ll in millón is usually pronounced like an English y sound in most of modern Spain
• ñ in año is like ny in canyon

So the most noticeable Spain feature here is:

• recibe ≈ re-THEE-be