Она уверяет нас, что всё готово.

Because уверять takes a direct object: уверять кого? → нас.

So the pattern is:

• уверять кого, что... = to assure someone that...
• уверять кого в чём = to assure someone of something

That is why Russian uses нас (accusative), not нам (dative).

Compare:

• Она уверяет нас, что всё готово.
• Она говорит нам, что всё готово.

With говорить, you normally get нам. With уверять, you get нас.

Here уверяет means assures.

It is stronger than just says. It suggests that she is telling us this confidently, possibly because we have doubts.

So она уверяет нас is closer to:

• she assures us
• she keeps assuring us
• she insists to us

depending on context.

The dictionary form is уверять.

уверяет is:

• 3rd person singular
• present tense
• imperfective

So it literally means she assures / is assuring.

Conjugation in the present:

• я уверяю
• ты уверяешь
• он/она уверяет
• мы уверяем
• вы уверяете
• они уверяют

Because что всё готово is a subordinate clause.

Russian normally puts a comma before что when it introduces a clause like that... after verbs such as:

• говорить
• думать
• знать
• считать
• уверять

So:

• Она уверяет нас, что всё готово.

This is standard Russian punctuation.

Here что means that and introduces what she is assuring us of.

So the structure is:

• Она уверяет нас = she assures us
• что всё готово = that everything is ready

This use of что is very common in Russian after verbs of speaking, thinking, knowing, and reporting.

Because готово here is a short-form adjective used as the predicate.

Russian often uses short-form adjectives after to be-type meanings:

• всё готово
• дом построен
• она довольна

In this sentence, всё is grammatically neuter singular, so the short form is готово.

Compare the short forms of готовый:

• готов — masculine
• готова — feminine
• готово — neuter
• готовы — plural

So:

• всё готово = everything is ready

No. It may look like an adverb because it ends in -о, but here it is not an adverb.

It is a short-form adjective agreeing with всё.

An adverb would describe a verb, like:

• Он говорит тихо. = He speaks quietly.

But in всё готово, готово describes the state of всё, so it is adjectival, not adverbial.

Because in Russian всё is grammatically neuter singular.

Even though the meaning is broad, grammatically it behaves like a singular pronoun. That is why the predicate is also singular neuter:

• всё готово
• всё ясно
• всё нормально

This is normal and very common in Russian.

Yes, in everyday writing Russians often omit the dots over ё, so you may see все instead of всё.

But in this sentence the meaning is clearly всё = everything, not все = everyone / all people.

So:

• всё готово = everything is ready
• все готовы = everyone is ready

The pronunciation is different, even if the spelling is sometimes simplified in real-life texts.

They are related, but not the same.

• уверять = to assure, to state confidently, to try to remove doubt
• убеждать = to persuade, to convince

So Она уверяет нас, что всё готово means she is telling us confidently that everything is ready.

If you said Она убеждает нас, что всё готово, that would sound more like she is trying to convince us of it, perhaps because we do not believe her.

It can mean either, depending on context.

Because уверять is imperfective, она уверяет can mean:

• she is assuring us now
• she keeps assuring us
• she habitually assures us

In a single sentence without context, English often translates it simply as she assures us.

Yes. Russian word order is flexible, though the neutral order here is very natural:

• Она уверяет нас, что всё готово.

Other orders are possible for emphasis, for example:

• Она нас уверяет, что всё готово.
  Slightly more emphasis on нас.
• Что всё готово, она уверяет нас.
  More literary or emphatic.

The basic meaning stays the same, but the emphasis shifts.

Sometimes it can be omitted if the subject is already clear from context:

• Уверяет нас, что всё готово.

But in many cases Russian keeps она to make the subject explicit.

Also, the verb form уверяет only tells you 3rd person singular, not the gender. So она helps specify that the subject is she rather than he.

A helpful stressed version is:

Она́ уверя́ет нас, что всё гото́во.

Approximate pronunciation:

• она → a-NA
• уверяет → oo-vye-RYA-yet
• нас → nas
• что → shto
• всё → roughly fsyo
• готово → ga-TO-va

A few useful points:

• что is usually pronounced што
• всё begins with a sound close to fs
• the stress in уверяет is on я

Sometimes in loose translation, yes, but assures is more accurate.

Говорит would be the straightforward verb for says:

• Она говорит нам, что всё готово.

But Она уверяет нас, что всё готово adds the idea that she is speaking reassuringly or insistently. So assures us is the better match.