A altitude aumentou rapidamente.

Because a is the definite article the.

In Portuguese, nouns usually appear with an article more often than in English. So A altitude means the altitude.

Also, altitude is a feminine noun, so it takes the feminine singular article a, not o.

You mainly learn the gender together with the noun: a altitude.

There is no perfect rule that lets you guess every noun’s gender, so it is best to memorise nouns with their article:

• a altitude
• a cidade
• o carro

Even though altitude ends in -e, it is still feminine.

Aumentou is the pretérito perfeito in European Portuguese, which corresponds to the simple past in English in this sentence.

It comes from the verb aumentar, meaning to increase or to go up.

So:

• aumentar = infinitive
• aumentou = increased / went up

This form is:

• 3rd person singular
• used with ele / ela / você
• and also with singular nouns like a altitude

Because the subject is singular: A altitude.

So the verb must also be singular:

• A altitude aumentou = singular subject + singular verb

Compare:

• A altitude aumentou = the altitude increased
• As altitudes aumentaram = the altitudes increased

And aumentava would be a different tense/aspect, usually meaning something like was increasing or used to increase, depending on context. Here, aumentou expresses a completed event.

Yes. They are related.

Both come from the idea of increase:

• aumentar = to increase
• aumento = increase, rise
• aumentou = increased

So if you know one, the others become easier to recognise.

Rapidamente is an adverb meaning quickly or rapidly.

It tells you how the altitude increased.

So the structure is:

• A altitude = subject
• aumentou = verb
• rapidamente = adverb of manner

A lot of Portuguese adverbs are formed with -mente, similar to English -ly:

• rápido = quick / fast
• rapidamente = quickly / rapidly

Note that the adjective is rápido, with an accent, but the adverb is rapidamente, without that accent.

Yes, sometimes.

The neutral, most straightforward order is:

• A altitude aumentou rapidamente.

But you may also hear:

• A altitude rapidamente aumentou.
• Rapidamente, a altitude aumentou.

These are all possible, but the original sentence is the most natural basic order for a learner.

In general, Portuguese allows a bit more flexibility than English, but not every rearrangement sounds equally natural.

They are related but not always interchangeable.

• altitude usually refers to altitude in the sense of height above sea level or vertical position in a technical/geographical/aeronautical sense.
• altura often means height in a broader sense, such as a person’s height or the height of an object.

Examples:

• A altitude do avião aumentou. = The plane’s altitude increased.
• A altura do prédio é impressionante. = The height of the building is impressive.

So in this sentence, altitude is the more specific and appropriate word.

In European Portuguese, aumentou is pronounced approximately like ow-men-TOH, but with Portuguese sounds, not English ones.

A few useful points:

• the final -ou is usually pronounced like a closed o sound, not like English ow
• the stress is on the last syllable: au-men-TOU
• in fast European Portuguese, unstressed vowels can sound reduced

So although English-style spelling help can be useful at first, listening to native audio is especially important for European Portuguese pronunciation.

A rough guide is:

A altitude aumentou rapidamente.
≈ uh al-ti-TOO-d(uh) ow-men-TOH ra-pi-da-MEN-t(uh)

A few notes:

• A often sounds like a very short uh
• altitude has stress on tu
• aumentou has stress on the last syllable
• rapidamente has stress on men
• final unstressed vowels in European Portuguese are often reduced

This is only an approximation. European Portuguese pronunciation is much more reduced and compressed than many learners expect.

Yes, depending on context.

Portuguese aumentar can often be translated in more than one natural way in English:

• increased
• rose
• went up
• got higher

The exact English wording depends on what the altitude refers to:

• a plane
• a mountain measurement
• a climbing route
• some technical reading

So the Portuguese itself is straightforward, but the best English translation may vary with context.