O termômetro mostra a temperatura.

These are the definite articles the in Portuguese.

• o = the for a singular masculine noun
• a = the for a singular feminine noun

So:

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

In Portuguese, articles are used more often than in English, so a sentence like this sounds very natural with both articles included.

In Portuguese, every noun has grammatical gender.

• termômetro is masculine, so it takes o
• temperatura is feminine, so it takes a

The ending often helps:

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

That is only a common pattern, not a perfect rule, but in this sentence it works exactly that way.

Mostra is the third person singular form of the verb mostrar in the present tense.

Conjugation:

• eu mostro = I show
• você / ele / ela mostra = you / he / she shows

Here, the subject is o termômetro, which is singular, so the verb is mostra:

• O termômetro mostra... = The thermometer shows...

Mostrar is the infinitive, meaning to show.

In a full sentence with a subject, you usually need a conjugated verb:

• mostrar = to show
• mostra = shows

So:

• O termômetro mostrar a temperatura is incorrect
• O termômetro mostra a temperatura is correct

It mainly means shows or displays in this context.

With instruments or devices, mostrar is very common:

• O relógio mostra a hora = The clock shows the time
• O painel mostra a velocidade = The dashboard shows the speed

So with termômetro, mostra sounds very natural.

Yes, but the meaning shifts slightly.

• mostra a temperatura = shows/displays the temperature
• mede a temperatura = measures the temperature

A thermometer does both in real life, but the focus changes:

• mostrar emphasizes what you can read or see
• medir emphasizes the act of measuring

So both can work, depending on what you want to express.

Because mostrar is a direct transitive verb here. It takes a direct object without a preposition.

• mostrar algo = to show something

So:

• mostra a temperatura = shows the temperature

You do not say mostra da temperatura in this structure.

The normal, most neutral order is:

• subject + verb + object
• O termômetro mostra a temperatura.

Portuguese does allow other orders in some contexts, especially for emphasis, but this standard order is the one learners should use first.

So this sentence is the most natural basic version.

Normally, no. That sounds incomplete or unnatural in standard Portuguese.

Portuguese usually wants the articles here:

• O termômetro mostra a temperatura.

Dropping articles can happen in headlines, labels, notes, or very telegraphic language, but not in a normal full sentence like this.

In Brazilian Portuguese, it is roughly:

tehr-MÔ-me-tro

A few helpful notes:

• the stress falls on mô
• ô is a closed o sound
• the final o in Brazilian Portuguese is often pronounced more like u in casual speech

So in natural Brazilian speech, it may sound a bit like:

tehr-MÔ-me-tru

In Brazilian Portuguese, it is roughly:

MÓS-tra

Notes:

• the stress is on the first syllable
• the o is open here
• the r in tra is soft because it is inside the word, not at the beginning

A rough Brazilian Portuguese pronunciation is:

tem-pe-ra-TU-ra

Notes:

• the stress falls on tu
• the final a is clearly pronounced
• each vowel is usually pronounced more clearly than in English

Yes, it is completely natural and correct.

It is a simple, standard sentence that a learner can safely use. It sounds like normal Brazilian Portuguese and is a good example of:

• article + noun
• present tense verb
• direct object

So it is a very useful model sentence.