Было ужасно тихо, когда я вернулась домой и открыла первую страницу.

Because this is an impersonal construction.

In Было ужасно тихо, there is no real grammatical subject. Russian often uses neuter singular past tense in this kind of sentence:

• Было холодно = It was cold
• Было темно = It was dark
• Было тихо = It was quiet

English uses a dummy subject it, but Russian usually does not.

Тихо here is not describing a noun directly. It describes the general state of the situation, so Russian uses a predicative adverb / category of state form.

Compare:

• Было тихо = It was quiet
• Комната была тихой = The room was quiet

In your sentence, the speaker is not saying that one specific noun was quiet. The whole atmosphere was quiet.

Here ужасно is an intensifier, meaning:

• terribly
• awfully
• extremely

So ужасно тихо means very / extremely quiet.

It does not necessarily mean something horrifying in a literal sense. Russian often uses words like ужасно, страшно, or жутко as intensifiers in everyday speech.

Вернулась is the past tense of вернуться, which means to return / to come back.

The -сь is the reflexive ending. In this verb, it is simply part of the normal dictionary form:

• вернуться = to return
• я вернулась = I returned / I came back

Also, вернулась is feminine past tense, so the speaker is female.

If the speaker were male, it would be:

• я вернулся

Because домой means to home / homeward, showing motion toward home.

• домой = home, to home
• дома = at home

So:

• я вернулась домой = I returned home
• я была дома = I was at home

English uses home for both ideas, but Russian separates them.

Because both actions are understood as completed single events:

• вернулась = returned
• открыла = opened

This fits the idea of a sequence in the past: first she came home, then she opened the first page.

If you used imperfective verbs, the meaning would change. It could sound more like an ongoing process, repetition, or background action.

For example:

• возвращалась = was returning / used to return
• открывала = was opening / used to open

In this sentence, perfective is the natural choice.

Because открыла takes a direct object, and direct objects are usually in the accusative case.

The noun is:

• страница = page

Its accusative singular form is:

• страницу

The adjective первая must agree with it, so it becomes:

• первую страницу

So the phrase means the first page in the accusative case.

Because когда я вернулась домой и открыла первую страницу is a subordinate clause introduced by когда.

Russian normally puts a comma between the main clause and a subordinate clause:

• Было ужасно тихо, когда...

This is similar to English punctuation in many cases.

Because и is simply joining two verbs with the same subject:

• я вернулась
• я открыла

The second я is omitted because it is understood.

So inside the subordinate clause, this is just one subject doing two actions, and no comma is needed before и.

Yes. Russian word order is fairly flexible.

You could also say:

• Когда я вернулась домой и открыла первую страницу, было ужасно тихо.

That is also correct.

The difference is mostly about focus and style:

• Было ужасно тихо, когда... puts the atmosphere first
• Когда..., было ужасно тихо puts the time-setting first

Both are natural.

Yes, я is understood.

Russian often omits a repeated subject when it is obvious:

• я вернулась домой и открыла первую страницу

This means:

• I returned home and opened the first page

Russian does not need to repeat я before the second verb unless the speaker wants extra emphasis.

Когда introduces the time when the state in the main clause was true.

So the structure is:

• Было ужасно тихо = main clause
• когда я вернулась домой и открыла первую страницу = time clause

In other words, the sentence means that the quietness existed at the moment when those actions happened.