A velocidade aumentou.

Because a here is the definite article meaning the.

So A velocidade = the speed.

In Portuguese, nouns usually need an article more often than in English. In this sentence, a velocidade sounds natural and complete.

Velocidade is a feminine noun, so it takes the feminine singular article a.

A useful pattern is that many nouns ending in -dade are feminine, such as:

• a cidade
• a verdade
• a velocidade

This is grammatical gender, so it does not mean the thing itself is biologically female.

Aumentou is the pretérito perfeito simples in Portuguese, which usually corresponds to the simple past in English.

It is the 3rd person singular form of aumentar.

So:

• eu aumentei
• tu aumentaste
• ele/ela/você aumentou

In this sentence, it means the increase happened as a completed event: the speed increased.

Because the subject is a velocidade, which is singular.

Portuguese verbs agree with the subject:

• A velocidade aumentou. = singular
• As velocidades aumentaram. = plural

Even though velocidade is feminine, that does not change the verb form here. The important thing for the verb is singular vs plural, not masculine vs feminine.

Yes. Aumentar can be used both intransitively and transitively.

In this sentence, it is intransitive:

• A velocidade aumentou.
  = The speed increased.

But it can also be transitive:

• O condutor aumentou a velocidade.
  = The driver increased the speed.

So the verb can describe something increasing by itself, or someone/something increasing it.

You could, but they do not sound as natural in the same way.

• A velocidade aumentou is the most direct and idiomatic way to say the speed increased.
• ficou maior sounds less natural with velocidade
• ficou mais rápida would more naturally describe something moving faster, not the abstract noun speed

So for this sentence, aumentou is the best choice.

Yes, that is possible, but it is less neutral.

• A velocidade aumentou. = the normal, straightforward order
• Aumentou a velocidade. = more marked; it may sound more literary, report-like, or focused on the event itself

For learners, the subject + verb order is the safest default.

A rough European Portuguese pronunciation is:

A velocidade aumentou
/ɐ vɨlusiˈðaðɨ o.mẽˈtow/

A few useful notes:

• velocidade has a softer, reduced vowel sound in unstressed syllables in European Portuguese
• the d in -dade often sounds like the th in this
• aumentou ends in a sound close to -toh, not -too

If you are used to Brazilian Portuguese, European Portuguese will often sound more reduced and compressed.

Because Portuguese usually expresses this meaning with the verb form itself.

English may use:

• The speed increased
• The speed has increased

Portuguese often just uses aumentou and lets context show whether English would use increased or has increased.

If you specifically want has increased, Portuguese can also say:

• A velocidade tem aumentado = The speed has been increasing
• A velocidade aumentou can still often cover the basic idea depending on context

The dictionary form is aumentar.

That is the infinitive, meaning to increase or to raise.

So when you look up aumentou, you should search for aumentar.

Yes, it is completely natural.

A velocidade aumentou. is a normal, correct sentence in European Portuguese. It could be used in contexts such as:

• driving
• physics
• machinery
• data or measurements
• news or technical reports

It is simple, neutral, and idiomatic.