A assistência e o reboque chegaram agora.

Because Portuguese nouns have grammatical gender.

• assistência is feminine, so it takes a
• reboque is masculine, so it takes o

This gender is grammatical, not necessarily related to biological sex. It is something you usually just learn with each noun.

So:

• a assistência
• o reboque

Because the subject is compound: a assistência e o reboque.

That means there are two things doing the action, so the verb must be plural:

• a assistência e o reboque chegaram

If there were only one subject, you would use singular:

• A assistência chegou agora.
• O reboque chegou agora.

In this kind of context, assistência usually does not mean abstract assistance/help in a general sense. It usually refers to the assistance service, especially something like roadside assistance.

So here, a assistência can mean the assistance team, company, or vehicle sent to help.

Portuguese often uses service nouns this way, where English might more naturally say something like the assistance team, roadside assistance, or the service depending on context.

Here, reboque refers to the tow service or tow truck, depending on context.

In everyday Brazilian Portuguese, people also often use guincho for tow truck. So depending on the situation, a Brazilian speaker might say either term, though usage can vary by region and context.

In this sentence, o reboque chegou is a natural way to refer to the towing help that arrived.

Because the noun itself is singular, even if it refers to a service made up of multiple people.

This is similar to English expressions like:

• The police arrived is plural in English
• but the service arrived would be singular

In Portuguese, assistência is grammatically a singular noun, so by itself it would take a singular verb:

• A assistência chegou

In your sentence, the verb becomes plural only because assistência is joined with o reboque.

With a past-tense verb like chegaram, agora usually means just now or a moment ago, not literally now at this exact second.

So in this sentence, agora is best understood as something like:

• just now
• a moment ago

That is a very common use in Brazilian Portuguese.

Portuguese often uses the simple past where English would naturally use either:

• arrived
• or have arrived

So chegaram agora is completely normal for something that happened very recently.

English often prefers have arrived just now or more naturally just arrived, but Portuguese does not need a separate present perfect form here.

Yes. Portuguese word order is fairly flexible, especially with adverbs like agora.

These are all possible, though they may sound slightly different in emphasis:

• A assistência e o reboque chegaram agora.
• Agora a assistência e o reboque chegaram.
• A assistência e o reboque agora chegaram.

The original version is very natural and neutral. It simply states that they arrived just now.

Usually, with this sentence, the articles a and o sound more natural.

• A assistência e o reboque chegaram agora.

Leaving them out can sound more like a headline, a label, or very telegraphic speech. In ordinary full sentences, Brazilian Portuguese often keeps the definite articles with nouns like these.

So for a learner, it is safest to use the articles here.

No. In a simple list of two coordinated nouns, Portuguese normally does not use a comma before e.

So this is correct:

• A assistência e o reboque chegaram agora.

That works the same way as standard English without a comma in a simple two-item subject.

A common Brazilian pronunciation is approximately:

• sheh-GAH-rãw

A few helpful notes:

• ch in Portuguese sounds like English sh
• the stressed syllable is ga
• the ending -am in this verb form is nasal, so it does not sound like English am

So the last part is more like a nasal ãw sound than a full am.

Yes, and it is very common.

• A assistência e o reboque chegaram agora.
• A assistência e o reboque acabaram de chegar.

Both can mean that they arrived very recently.

The difference is mostly one of style and nuance:

• chegaram agora = arrived just now
• acabaram de chegar = have just arrived

Both are natural in Brazilian Portuguese.