O contêiner está cheio.

Use estar for a current state or condition. A container being full is normally seen as a temporary condition, so está cheio is the natural choice.

Using ser here would sound unusual, because ser is generally used for more permanent characteristics or definitions.

O is the masculine singular definite article, meaning the.

Portuguese uses articles more often than English does, so O contêiner is the normal way to say the container. If you wanted a container, you would say um contêiner.

Because cheio has to agree with contêiner, and contêiner is masculine singular.

In Portuguese, adjectives usually match the noun in gender and number:

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

The acute accent in está shows that the stress falls on the last syllable: es-TÁ.

It also helps distinguish this verb form from esta, which without the accent can be a different word in other contexts.

The circumflex in ê helps mark the pronunciation and stress. The stressed part is têi.

So the word is stressed roughly like this: con-TÊI-ner.

A rough English-friendly guide is con-TAY-ner, with the stress on the middle syllable.

That said, it is not exactly like the English word container. In Brazilian Portuguese:

• the vowels are different
• the stress is clearly on têi
• the final r is often softer or breathier than an English r, depending on the accent

No. O contêiner está cheio is a complete sentence by itself if the context already makes things clear.

If you want to mention the contents, use cheio de:

• O contêiner está cheio de água.
• O contêiner está cheio de caixas.

Not in this sentence structure.

Here, cheio is part of estar + adjective:

• O contêiner está cheio.

You would not normally say O cheio contêiner está.

More generally, Portuguese adjectives often come after the noun, but in this sentence the adjective comes after the verb because it describes the container’s state.

Usually no.

Lotado is more commonly used for places or vehicles crowded with people, such as:

• O ônibus está lotado.
• O metrô está lotado.

For a container being full, cheio is the normal and natural choice.

You would say:

Os contêineres estão cheios.

Notice how several words change to match the plural:

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

Yes, contêiner is a standard word in Brazilian Portuguese, especially for a shipping container or a large container in general.

It is a borrowed word, which is why it may look similar to English, but its spelling and pronunciation have been adapted to Portuguese.