O cantil está cheio?

O means the, while um means a/an.

So o cantil refers to a specific canteen/flask that both speaker and listener already know about.

• O cantil está cheio? = Is the canteen full?
• Um cantil está cheio? would sound more like Is a canteen full?, which is much less natural in most situations.

Cantil usually means a canteen or flask, especially the kind used for hiking, camping, military use, or carrying water.

In everyday Brazilian Portuguese, people might also use other words depending on the object:

• garrafa = bottle
• garrafinha = small bottle
• squeeze = sports water bottle

So cantil is correct, but it refers to a fairly specific type of container.

Portuguese has two main verbs for to be: ser and estar.

Here, está is used because being full is seen as a state or condition, not a permanent characteristic.

• O cantil está cheio. = The canteen is full.
• está = temporary/current state

Using é here would sound unnatural, because full is not an essential identity of the canteen.

The accent in está shows the stressed syllable and is part of the correct spelling.

• está = is
• esta without the accent usually means this in the feminine form

So the accent is important because it distinguishes different words.

Because cheio has to agree with the noun it describes.

Cantil is a masculine noun, so the adjective is masculine too:

• o cantil está cheio

If the noun were feminine, you would use cheia:

• a garrafa está cheia = the bottle is full

Agreement is very important in Portuguese.

In Portuguese, yes/no questions often keep the same word order as statements. The main difference is usually:

• intonation in speech
• a question mark in writing

So:

• O cantil está cheio. = The canteen is full.
• O cantil está cheio? = Is the canteen full?

Unlike English, Portuguese usually does not need a word-order change like Is the canteen full?

Yes, it is possible, but it is less neutral and can sound more formal, literary, or marked for emphasis.

The most natural everyday version is:

• O cantil está cheio?

So as a learner, this is the form you should normally use.

Usually, no, not in normal standard speech.

In Portuguese, nouns often need an article when they are the subject of the sentence:

• O cantil está cheio? sounds natural.
• Cantil está cheio? sounds incomplete or unnatural in most contexts.

You might hear article dropping in very specific styles, labels, headlines, or very informal speech, but it is not the standard pattern to copy as a beginner.

A rough pronunciation guide for an English speaker is:

oo kahn-CHEEW es-TAH SHAY-oo?

A few notes:

• o is often pronounced like oo
• til in cantil sounds close to cheew in Brazilian Portuguese
• está has stress on the last syllable: -TA
• cheio sounds roughly like SHAY-oo

Pronunciation varies by region, but this is a useful general approximation.

Yes. Very often, if you want to say what something is full of, you use cheio de.

Examples:

• O cantil está cheio de água. = The canteen is full of water.
• A mochila está cheia de coisas. = The backpack is full of things.

In your sentence, de + noun is simply omitted because the question only asks whether it is full.

Cantil is definitely a real and correct word in Brazil, but it is more associated with:

• camping
• hiking
• military gear
• an old-fashioned or specific type of water container

If you mean an ordinary bottle, many Brazilians would more naturally say:

• garrafa
• garrafinha
• squeeze for a sports bottle

So cantil is fine, but the most natural choice depends on the object you mean.

The plural would be:

• Os cantis estão cheios? = Are the canteens full?

Notice the changes:

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

Everything has to agree in number.