O avião chegou hoje.

O is the masculine singular definite article, meaning the.

So:

• o avião = the airplane / the plane

In Portuguese, articles are used more often than in English, so o avião sounds natural where English might sometimes just say the plane or even simply plane in some contexts.

Because avião is a masculine noun, it takes o, not a.

You usually learn the gender together with the noun:

• o avião = the airplane

There is no perfect rule that tells you the gender of every noun just from its form, although some endings give hints. The safest habit is to memorize nouns with their article:

• o avião
• a casa
• o carro
• a cidade

That way, you automatically learn the correct gender.

The ã shows a nasal sound.

So avião is not pronounced like a plain a sound. The stressed part is nasal, roughly like ah-VYOWN, though English spelling cannot represent it perfectly.

A few useful points:

• ã = nasal vowel
• the final -ão is a very common ending in Portuguese
• avião is pronounced approximately ah-vee-OWN with a nasal ending

Also, the written accent helps show the stressed syllable.

The accent mark in avião helps indicate stress and is part of the correct spelling.

In this word, the stress falls on the final syllable:

• a-vi-ÃO

Portuguese uses accent marks much more systematically than English. They can show:

• which syllable is stressed
• vowel quality
• sometimes irregular stress patterns

So the accent is not optional; it is part of the word.

Chegou is the simple past form of the verb chegar (to arrive).

• chegar = to arrive
• chegou = arrived

Specifically, it is the third person singular form of the pretérito perfeito.

Examples:

• O avião chegou. = The plane arrived.
• Ela chegou cedo. = She arrived early.
• Meu amigo chegou ontem. = My friend arrived yesterday.

Because chegar here is being used normally, with a straightforward subject:

• O avião = the subject
• chegou = arrived

So:

• O avião chegou hoje. = The plane arrived today.

You do not need se here.
The verb simply agrees with the subject in third person singular:

• o avião chegou
• os aviões chegaram

Portuguese usually does not need a subject pronoun when the subject is already stated.

Here, the subject is explicitly named:

• O avião

So there is no need to add a pronoun like ele.

In fact, saying O avião ele chegou hoje would generally sound unnatural in a normal neutral sentence.

Portuguese often avoids unnecessary subject pronouns, especially when the subject noun is already there.

Yes, depending on context, chegou can sometimes correspond to either:

• arrived
• has arrived

Portuguese commonly uses the simple past (pretérito perfeito) in places where English might use either the simple past or the present perfect.

So O avião chegou hoje could be translated as:

• The plane arrived today
• The plane has arrived today

The best English choice depends on context and style.

Hoje means today, and putting time expressions at the end is very common in Portuguese.

So:

• O avião chegou hoje. = The plane arrived today.

But Portuguese is flexible. You can also say:

• Hoje o avião chegou.

That changes the emphasis slightly, putting more focus on today.

The version with hoje at the end is very neutral and natural.

Usually, in standard neutral Portuguese, you would say O avião chegou hoje.

Leaving out the article and saying Avião chegou hoje is generally not the normal choice in this context. It may sound telegraphic, headline-like, or unnatural in ordinary speech.

Portuguese often prefers articles before nouns more than English does.

So for a normal sentence, use:

• O avião chegou hoje.

Approximately, chegou sounds like sheh-GOH in much of Brazil, though this is only an approximation.

A few pronunciation details:

• ch in Brazilian Portuguese usually sounds like sh
• g before ou sounds like a hard g, as in go
• the stress is on the last syllable: che-GOU

So:

• chegou ≈ sheh-GOH

Pronunciation varies somewhat by region, but this is a good general Brazilian approximation.

The infinitive is chegar (to arrive).

Here is the present tense:

• eu chego = I arrive
• você/ele/ela chega = you/he/she arrives
• nós chegamos = we arrive
• vocês/eles/elas chegam = you/they arrive

And the simple past relevant to this sentence:

• eu cheguei = I arrived
• você/ele/ela chegou = you/he/she arrived
• nós chegamos = we arrived
• vocês/eles/elas chegaram = you/they arrived

Notice that chegamos can mean either we arrive or we arrived; context tells you which one is meant.

The verb would need to agree with the plural subject.

Singular:

• O avião chegou hoje. = The plane arrived today.

Plural:

• Os aviões chegaram hoje. = The planes arrived today.

Changes:

• o → os
• avião → aviões
• chegou → chegaram

This is a very important pattern in Portuguese: the verb changes to match the subject.

Yes, avião is the normal everyday word for airplane / plane in Brazilian Portuguese.

You may also see:

• aeronave = aircraft
  This is more formal or technical.

In normal conversation, avião is the standard word people use.