O termômetro mede a temperatura.

Because Portuguese nouns have grammatical gender.

• termômetro is a masculine noun, so it takes o (the).
• temperatura is a feminine noun, so it takes a (the).

So:

• o termômetro = the thermometer
• a temperatura = the temperature

This gender is a property of the noun itself and does not usually mean the object is literally male or female.

Mede is the third-person singular form of the verb medir (to measure) in the present tense.

Here is the pattern:

• eu meço = I measure
• você / ele / ela mede = you / he / she / it measures

Since o termômetro is a singular subject (the thermometer), Portuguese uses mede.

So:

• O termômetro mede... = The thermometer measures...

The infinitive is medir, which means to measure.

In this sentence:

• mede = measures
• medir = to measure

So if you wanted to say A thermometer can measure temperature, you would use the infinitive medir in a different structure.

Portuguese often does not use an explicit subject pronoun when the subject is already stated.

In English, you might think of:

• The thermometer measures the temperature
• or It measures the temperature

But in Portuguese, once the noun is there, you just say:

• O termômetro mede a temperatura.

There is no need to add a separate word for it.

Yes, in this sentence the word order is basically the same:

• O termômetro = the subject
• mede = the verb
• a temperatura = the object

So it follows:

Subject + Verb + Object

This is one reason the sentence feels fairly straightforward to English speakers.

In normal Portuguese, articles are used more often than in English, especially with everyday concrete nouns.

So the natural sentence is:

• O termômetro mede a temperatura.

Leaving out the articles:

• termômetro mede temperatura

would sound unnatural or incomplete in standard everyday Portuguese.

Portuguese often prefers:

• the thermometer
• the temperature

even where English might sometimes be more flexible.

No. Temperatura is the general word for temperature.

It can refer to:

• body temperature
• room temperature
• outside temperature
• water temperature

So in this sentence, a temperatura is general unless the context makes it specific.

In Brazilian Portuguese, termômetro is pronounced approximately like:

tehr-MOH-meh-troo

A few helpful points:

• The stressed syllable is mô.
• The ô shows a closed o sound.
• The final o in Brazilian Portuguese often sounds closer to oo in unstressed position.

So the rhythm is:

ter-MÔ-me-tro

The circumflex accent (ô) helps show the stressed syllable and vowel quality.

In termômetro:

• the stress falls on mô
• ô indicates a more closed o sound

Without the accent, the pronunciation rules would suggest a different stress pattern, so the accent is important.

Not in this sentence.

Medir literally means to measure.

Portuguese can use medir a temperatura for measure the temperature. In English, we often also say:

• take someone’s temperature

But Portuguese does not use tomar here in the same direct way for a thermometer doing the action.

So:

• O termômetro mede a temperatura. = The thermometer measures the temperature.

That is the most direct and natural match.

Yes.

• O termômetro mede a temperatura. = The thermometer measures the temperature.
• Um termômetro mede a temperatura. = A thermometer measures the temperature.

Both are grammatical, but they mean slightly different things.

• o points to a specific thermometer, or can sound generic in a class-of-things way
• um means a / one, introducing a nonspecific thermometer

The ending -o often suggests a masculine noun, and in this case that is true:

• o termômetro

But it is better to think of the article and noun together when learning vocabulary, because endings are helpful clues, not absolute rules.

For example:

• many nouns ending in -o are masculine
• many nouns ending in -a are feminine

But there are exceptions in Portuguese, so memorizing o termômetro and a temperatura is the safest approach.

Yes. Medir can be used with people, objects, distances, and more.

For example:

• O médico mede a pressão. = The doctor measures the pressure.
• Ela mede dois metros. = She is two meters tall.
• Eu medi a mesa. = I measured the table.

So medir is a common and flexible verb.

Because medir is a direct transitive verb here: it takes a direct object without a preposition.

So:

• mede a temperatura = measures the temperature

You do not say:

• mede de a temperatura
• mede para a temperatura

Just use the verb directly with the thing being measured.

Yes. The present tense in Portuguese can express a general truth or fact.

So:

• O termômetro mede a temperatura.

can mean something like:

• A thermometer measures temperature
• The thermometer measures the temperature

Depending on context, it can describe:

• what a thermometer does in general
• what a specific thermometer is doing or is designed to do

That kind of flexibility is very common in the Portuguese present tense.