O mecânico está na oficina.

O means the and is the masculine singular definite article.

• o mecânico = the mechanic
• a mecânica = the female mechanic

Portuguese uses articles very often, sometimes more often than English. In this sentence, o tells you we are talking about a specific mechanic, not just any mechanic.

If you wanted a mechanic, you would say:

• Um mecânico está na oficina. = A mechanic is in the workshop/garage.

Because mecânico here refers to a male mechanic, and the noun is masculine.

• o mecânico = the male mechanic
• a mecânica = the female mechanic

Also, be careful: mecânica can also mean mechanics as a field or mechanical system, depending on context. But in this sentence, mecânico clearly means the person.

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

Here, está is used because the sentence talks about location:

• O mecânico está na oficina. = The mechanic is in the workshop.

A very common rule for beginners is:

• ser = identity, permanent characteristics, profession, origin, time
• estar = location, condition, temporary states

So:

• Ele é mecânico. = He is a mechanic.
• Ele está na oficina. = He is in the workshop.

Na is a contraction of:

• em
  • a = na

So:

• na oficina literally = in the workshop
• em a oficina is not used; it contracts to na

Other common contractions are:

• no = em + o
• nos = em + os
• nas = em + as

Examples:

• no carro = in the car
• na casa = in the house
• nos livros = in the books
• nas ruas = in the streets

Because oficina is being used with the definite article a, so em + a becomes na.

Portuguese often uses articles where English does too, and sometimes where English does not. In this sentence, na oficina means in the workshop/the garage.

You might hear em uma oficina if the meaning is in a workshop/garage:

• O mecânico está em uma oficina. = The mechanic is in a workshop.

But na oficina suggests a specific one, or one already known from context.

Oficina usually means workshop, repair shop, or garage, depending on context.

With mecânico, the most natural English translation is often:

• garage
• repair shop
• workshop

So O mecânico está na oficina could be understood as:

• The mechanic is in the workshop.
• The mechanic is at the garage/repair shop.

The exact English word depends on context.

In Brazilian Portuguese, mecânico is pronounced roughly like:

• meh-KAH-nee-koo

The accent mark in mecânico shows the stress goes on câ:

• me-CÂ-ni-co

A few pronunciation notes:

• e in me is usually like meh
• câ has the stressed syllable
• final -co is pronounced clearly, not silent

The accent mark in mecânico shows the stressed syllable and also helps indicate vowel quality.

Without getting too technical, it tells you that the stress falls here:

• me-CÂ-ni-co

Portuguese spelling uses accent marks much more than English does, and they are important. They can change pronunciation and sometimes meaning.

So when learning vocabulary, it is best to memorize the accent together with the word.

Yes, in this sentence the word order is very similar to English:

• O mecânico = the mechanic
• está = is
• na oficina = in the workshop

So it follows the pattern:

• subject + verb + place

This makes it a very beginner-friendly sentence.

No. In standard Portuguese, em a contracts to na.

So the correct form is:

• O mecânico está na oficina.

This kind of contraction is very common and important in Portuguese.

Normally, no. In Portuguese, countable singular nouns usually need an article or some determiner.

So these are natural:

• O mecânico está na oficina. = The mechanic is in the workshop.
• Um mecânico está na oficina. = A mechanic is in the workshop.

But Mecânico está na oficina sounds incomplete in standard Portuguese.

You would change the article and the noun to the feminine form:

• A mecânica está na oficina.

That means The female mechanic is in the workshop.

Notice:

• o → masculine singular
• a → feminine singular
• mecânico → masculine form
• mecânica → feminine form

The rest of the sentence stays the same.

Yes. In many real situations, English may translate it as either in the workshop or at the workshop, depending on what sounds more natural.

Portuguese em / no / na often covers meanings that English expresses with either in or at.

So:

• O mecânico está na oficina.

could be translated naturally as:

• The mechanic is in the workshop.
• The mechanic is at the garage.

Both can work, depending on context.