O autocarro chega ao mercado ao meio-dia.

Chega is the present tense, 3rd person singular form of the verb chegar (to arrive).

• chegar = the infinitive (to arrive)
• chega = he/she/it arrives (or “the bus arrives”)
• chegou = he/she/it arrived (simple past)

In Portuguese (as in English), we conjugate the verb to match the subject:

• O autocarro chega = The bus arrives
• Os autocarros chegam = The buses arrive

Here the subject o autocarro is singular, so we use chega.

Portuguese uses definite articles (o, a, os, as) much more often than English, especially with common nouns.

• O autocarro = literally “the bus”
• Just autocarro chega… would sound incomplete or wrong in standard Portuguese.

In English we can often drop the (“Buses arrive at noon”), but in Portuguese a singular, specific thing (like o autocarro on a schedule) normally takes the definite article.

Every Portuguese noun has grammatical gender, and autocarro is masculine.

• Masculine singular article: o
• Feminine singular article: a

You simply have to learn the gender with each noun:

• o autocarro (the bus) – masculine
• a estação (the station) – feminine

So we say o autocarro, not a autocarro.

Portuguese often uses the present tense for scheduled future events, just like English does:

• English: “The bus arrives at noon.” (present form, future meaning)
• Portuguese: O autocarro chega ao mercado ao meio-dia.

You could also use the future tense chegará:

• O autocarro chegará ao mercado ao meio-dia.

That is grammatically correct but sounds more formal or written. In timetables, announcements, and everyday speech, the present chega is very natural for fixed schedules.

Ao is a contraction of the preposition a plus the masculine singular article o:

• a + o = ao

So:

• ao mercado = a (to/at) + o mercado (the market)
• ao meio-dia = a (at) + o meio-dia (the midday / noon)

When a comes before o (masculine singular “the”), they normally combine into ao.

The verb chegar normally takes the preposition a to express destination:

• chegar a algum lugar = to arrive at / in a place

So you get:

• chegar ao mercado = arrive at the market

No is em + o (“in/on + the”), and chegar em is more typical of Brazilian Portuguese in informal speech. In European Portuguese, chegar a is standard and preferred.

So in Portugal:

• O autocarro chega ao mercado… is the natural choice.

Chegar combines with a, not with para, to talk about the place you reach:

• chegar a Lisboa / ao mercado / a casa

Para often expresses direction or purpose (“towards / in order to”):

• Vou para o mercado. = I’m going (towards) the market.
• O autocarro vai para o mercado. = The bus goes to the market.

But when you finally arrive, you say:

• O autocarro chega ao mercado.

Roughly:

• ao mercado (with a) after chegar = the place where you end up, where you arrive.
• para o mercado (with para) usually after ir, vir, seguir, etc. = direction or destination of a movement.

Examples:

• O autocarro vai para o mercado.
  The bus is going to the market (its route/direction).
• O autocarro chega ao mercado.
  The bus arrives at the market (it has reached it).

Both are possible, but they are slightly different:

• ao meio-dia = at noon
• às doze = at twelve (o’clock)

Meio-dia is a specific noun (“midday/noon”), and it takes the masculine article:

• o meio-dia → ao meio-dia

The expression às doze comes from:

• a + as doze horas (to/at + the twelve hours)
• a + as → às So:
• às doze (horas) = at twelve (o’clock)

In your sentence, ao meio-dia is the more idiomatic way to say “at noon.”

Yes, in modern orthography it is normally written with a hyphen:

• meio-dia (noon)
• meia-noite (midnight)

So the standard form is:

• ao meio-dia, not ao meio dia.

Yes, grammatically that is fine and would mean “The bus arrives at the market at twelve (o’clock).”

Nuance:

• ao meio-dia sounds like exactly noon.
• às doze is more neutral “at twelve o’clock” (which is also noon, of course), and might feel a bit more “clock-time” style.

In everyday Portuguese, ao meio-dia is very natural for “at noon.”

Yes, that word order is also correct:

• O autocarro chega ao mercado ao meio-dia.
• Ao meio-dia, o autocarro chega ao mercado.

Putting the time expression ao meio-dia at the beginning emphasizes the time. It is similar to English:

• “At noon, the bus arrives at the market.”

What you normally would not do is split the verb and its complements in a strange way, like:

• O autocarro ao meio-dia chega ao mercado.
  This is possible but feels awkward and is not the usual ordering.

In Brazil, the usual word for “bus” is ônibus, not autocarro.

A natural Brazilian Portuguese version would be:

• O ônibus chega ao mercado ao meio-dia.

Other points:

• In Brazil, many people say chegar no mercado instead of chegar ao mercado, although chegar ao mercado is also correct and more formal.
• The rest (ao meio-dia) is the same.