A realidade mudou rapidamente.

Because a is the feminine singular definite article, equivalent to the.

So a realidade = the reality.

Portuguese uses articles more often than English, including with abstract nouns. That means A realidade mudou rapidamente sounds very natural, even if English might sometimes just say Reality changed quickly without the.

Yes, realidade is a feminine noun: a realidade.

A useful pattern is that many nouns ending in -dade are feminine, for example:

• a cidade
• a liberdade
• a verdade
• a realidade

That pattern is helpful, but the safest habit is to learn nouns together with their article: a realidade, not just realidade.

Mudou is the 3rd person singular form of the preterite (simple past) of mudar.

It is used with:

• ele mudou
• ela mudou
• você mudou
• a realidade mudou

So the verb matches the singular subject a realidade.

Because mudou presents the change as a completed event.

• mudou = changed
• mudava = was changing / used to change / would change habitually, depending on context

In this sentence, the speaker is treating the change as something that happened, not as an ongoing background process. That is why mudou is the natural choice.

No. Here mudar is being used intransitively, which means the subject itself changes.

So:

• A realidade mudou. = complete sentence

Nothing is missing.

Compare:

• A realidade mudou. → reality changed
• A crise mudou a realidade. → the crisis changed reality

In the second example, mudar is transitive because it has an object: a realidade.

Because that is a very normal position for adverbs in Portuguese.

A neutral word order is:

• subject + verb + adverb

So:

• A realidade mudou rapidamente.

sounds natural and standard.

You can move the adverb for emphasis, for example:

• Rapidamente, a realidade mudou.
• A realidade rapidamente mudou.

But those versions feel more marked or stylistic. The original order is the most neutral.

Rapidamente is formed from the adjective rápida + -mente.

A very common rule in Portuguese is:

• adjective in the feminine singular form + -mente → adverb

So:

• rápida
  • mente → rapidamente

It is not rápidomente because -mente attaches to the feminine form.

It is also not rápidamente in Portuguese spelling. When this adverb is formed, the written accent from rápida disappears, giving rapidamente.

Yes.

These are all possible:

• A realidade mudou rapidamente.
• A realidade mudou depressa.
• A realidade mudou rápido.

The difference is mostly style:

• rapidamente = a bit more formal or written
• depressa = very natural and common
• rápido used as an adverb = common in everyday Brazilian Portuguese, a bit more informal

So all three can work, but rapidamente sounds slightly more formal.

Usually, no—not in a normal full sentence.

A realidade mudou rapidamente is the natural standard version.

If you drop the article, it can sound like:

• a headline
• a title
• a note
• very compressed or telegraphic language

So in ordinary speech and writing, keep a.

A useful stress guide is:

a realiDAde muDOU rapidaMENte

A rough English-style approximation is:

• A realidade → ah heh-ah-lee-DAH-jee
• mudou → moo-DOH
• rapidamente → hah-pee-dah-MEN-chee

A few pronunciation notes:

• In many Brazilian accents, word-initial r sounds somewhat like an English h.
• In many regions of Brazil, de and te at the end of syllables can sound like jee and chee.
• The strongest stresses are on -da-, -dou, and -men-.