O ônibus está cheio hoje.

Because cheio here describes a temporary condition: the bus is full today, at this moment or in this situation.

In Portuguese, estar is usually used for temporary states or conditions:

• O ônibus está cheio hoje. = The bus is full today.

Ser would sound unusual here because it is more often used for permanent characteristics or definitions. In everyday Portuguese, when talking about a bus, train, restaurant, etc. being crowded or full at a certain time, estar is the normal choice.

Here, cheio means full or crowded, depending on context.

With ônibus, it usually means the bus is full of passengers, so in natural English the idea is often:

• The bus is full today
• or The bus is crowded today

Literally, cheio means full.

O is the masculine singular definite article, meaning the.

So:

• o ônibus = the bus

In Portuguese, articles are used more often than in English. A sentence like Ônibus está cheio hoje would normally sound incomplete or unnatural in standard usage unless it appeared in a very specific context, like a headline or note.

In this sentence, ônibus is singular.

That is one tricky thing about this word:
ônibus has the same form in both singular and plural.

So:

• o ônibus = the bus
• os ônibus = the buses

You can tell whether it is singular or plural by the article or the rest of the sentence:

• O ônibus está cheio. = The bus is full.
• Os ônibus estão cheios. = The buses are full.

The accent mark shows the stressed syllable and helps indicate pronunciation.

ônibus is pronounced roughly like:

• OH-nee-boos

The stress falls on the first syllable: Ô.

Without getting too technical, the accent helps show that the word does not follow the most basic default stress pattern.

Because cheio agrees with ônibus, and ônibus is a masculine noun.

In Portuguese, adjectives usually agree in gender and number with the noun they describe:

• masculine singular: cheio
• feminine singular: cheia
• masculine plural: cheios
• feminine plural: cheias

Examples:

• O ônibus está cheio.
• A sala está cheia.
• Os ônibus estão cheios.
• As salas estão cheias.

Yes, hoje can go in different positions.

The sentence:

• O ônibus está cheio hoje.

is completely natural and common.

You could also say:

• Hoje o ônibus está cheio.

Both are correct. The difference is mainly one of emphasis or rhythm:

• O ônibus está cheio hoje. emphasizes the situation of the bus, then adds today.
• Hoje o ônibus está cheio. highlights today right away.

A simple pronunciation guide is:

O ônibus está cheio hoje
≈ oo OH-nee-boos es-TAH SHAY-oo OH-zhee

A few notes:

• está has stress on the last syllable: -tá
• cheio sounds roughly like SHAY-oo
• in most of Brazil, hoje sounds like OH-zhee or OH-zhee/zhih

This is only an approximation using English spelling, but it can help at first.

Yes, very often.

Cheio can describe many things that are full:

• O copo está cheio. = The glass is full.
• A sala está cheia. = The room is full.
• O metrô está cheio. = The subway is crowded/full.
• O restaurante está cheio. = The restaurant is full/crowded.

So this is a very useful everyday adjective.

You would say:

Os ônibus estão cheios hoje.

Notice all the plural changes:

• o → os
• está → estão
• cheio → cheios

But ônibus stays the same:

• singular: ônibus
• plural: ônibus