Когда сайт наконец загрузился, я сразу заметила предупреждение о переносе встречи.

Because Когда сайт наконец загрузился is a subordinate clause introduced by когда = when.

In Russian, a subordinate clause is normally separated from the main clause by a comma:

Когда сайт наконец загрузился, я сразу заметила предупреждение...

So the structure is:

• Когда сайт наконец загрузился = when the site finally loaded
• я сразу заметила... = I immediately noticed...

Even if the English translation might not always feel like it needs a pause, Russian punctuation does.

Загрузился comes from the verb загрузиться, which means to load or to finish loading in contexts like websites, pages, apps, files, etc.

The ending -ся often makes a verb intransitive or gives it a meaning like to become loaded / to load up.

So:

• Я загрузил файл = I uploaded/loaded the file
• Сайт загрузился = the site loaded

Here, the site is not loading something else; it itself finishes loading. That is why загрузился is the natural form.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is сайт, which is a masculine noun. So the verb is masculine past:

• сайт загрузился = the site loaded

Compare:

• страница загрузилась = the page loaded
• окно загрузилось = the window loaded
• сайты загрузились = the sites loaded

Because the speaker is female.

In Russian past tense, the verb agrees with the gender of the person doing the action:

• я заметил = I noticed (male speaker)
• я заметила = I noticed (female speaker)

So this sentence tells you something about the speaker: she is female.

Наконец here means finally.

It modifies the idea of the site loading, so it is placed in the first clause:

Когда сайт наконец загрузился... = When the site finally loaded...

Russian word order is flexible, so you could move it for emphasis, but this position is very natural. For example:

• Когда наконец сайт загрузился...
• Когда сайт загрузился наконец...

These are possible, but Когда сайт наконец загрузился sounds the most neutral and natural.

Сразу means immediately, right away, or at once.

In this sentence it modifies заметила:

я сразу заметила = I immediately noticed

Placing сразу before the verb is very common and neutral in Russian. You could move it, but that changes emphasis slightly:

• Я сразу заметила предупреждение = neutral, natural
• Я заметила предупреждение сразу = more emphasis on how quickly it happened

It is in the accusative, but for this noun the accusative looks exactly like the nominative.

Предупреждение is a neuter inanimate noun. For many neuter inanimate nouns, nominative and accusative singular are the same:

• nominative: предупреждение
• accusative: предупреждение

Since заметить takes a direct object, we do need the accusative:

• заметить что? → предупреждение

It just happens that the form does not change.

Because the noun предупреждение often takes the preposition о when you say what the warning is about.

So:

• предупреждение о чём? = warning about what?
• о переносе встречи = about the rescheduling/postponement of the meeting

This uses the prepositional case, so:

• перенос → о переносе

A few examples:

• сообщение о проблеме = a message about a problem
• новость о концерте = news about the concert
• предупреждение о шторме = a warning about a storm

Because перенос встречи means the rescheduling/postponement of the meeting, and after the noun перенос the thing being moved is usually put in the genitive case.

So:

• перенос чего? = rescheduling/postponement of what?
• встречи = of the meeting

Base form:

• встреча = meeting

Genitive singular:

• встречи

So the whole phrase works like this:

• предупреждение о переносе встречи
• warning about the rescheduling of the meeting

Because the sentence describes completed events in sequence.

• загрузился is perfective: the site finished loading
• заметила is perfective: she noticed the warning as a completed action

This gives a clear timeline:

1. the site finished loading
2. she immediately noticed the warning

If you used imperfective forms, the meaning would change. For example, imperfective often suggests process, repetition, or background action rather than a single completed event.

So the perfective aspect is the natural choice here.

Usually not if you want the same meaning.

Когда сайт загружался... means while the site was loading... That describes an ongoing process, not the completed moment when the site had finished loading.

So compare:

• Когда сайт загрузился, я сразу заметила...
  = When the site loaded / had loaded, I immediately noticed...

• Когда сайт загружался, я...
  = While the site was loading, I...

The original sentence needs the completed event, so загрузился is correct.

No. They are different.

• перенос встречи = the meeting was moved to another time/date
• отмена встречи = the meeting was cancelled

So this warning is not saying the meeting disappeared; it is saying it was rescheduled or postponed.

The given word order is very natural and neutral:

Когда сайт наконец загрузился, я сразу заметила предупреждение о переносе встречи.

Russian word order is flexible, but different orders change emphasis. For example:

• Я сразу заметила предупреждение о переносе встречи, когда сайт наконец загрузился.
  Still understandable, but the original sounds smoother if you want to set the time first.

• Когда сайт наконец загрузился, предупреждение о переносе встречи я заметила сразу.
  More marked; it emphasizes предупреждение and сразу.

For learners, the original order is a very good model of normal Russian.

It is:

Когда + clause, main clause

More specifically:

• Когда сайт наконец загрузился
  subordinate time clause

• я сразу заметила предупреждение о переносе встречи
  main clause

Inside the main clause:

• я = subject
• заметила = verb
• предупреждение = direct object
• о переносе встречи = phrase explaining what the warning was about

So the sentence is built very logically, even though some of the case endings may look complicated at first.