Жалобу отправили вчера.

Because жалобу is in the accusative case, not the nominative.

• жалоба = nominative singular, the basic dictionary form
• жалобу = accusative singular

In this sentence, жалобу is the direct object of the verb отправили: it is the thing that was sent.

For feminine nouns ending in -а, the accusative singular usually changes to -у:

• жалоба → жалобу
• книга → книгу
• машина → машину

So the sentence uses жалобу because Russian marks the object grammatically.

This is a very common Russian pattern.

Отправили is the past tense plural form of отправить. Literally, it looks like they sent. But in Russian, a plural verb can be used with no stated subject to mean something like:

• they sent it
• someone sent it
• it was sent

This is called an indefinite-personal construction. The speaker does not name who did the action, either because it is unknown, unimportant, or obvious from context.

So:

• Жалобу отправили вчера.

can naturally mean:

• They sent the complaint yesterday
• or, in more natural English, The complaint was sent yesterday

Russian often prefers this kind of active-looking sentence where English might prefer the passive.

Yes. A more directly passive version would be:

• Жалоба была отправлена вчера.

That literally corresponds to The complaint was sent yesterday.

Compare the two:

• Жалобу отправили вчера.
  an indefinite they did it; very natural in everyday Russian

• Жалоба была отправлена вчера.
  more explicitly passive

Both are correct, but the first one is extremely common in spoken and written Russian when the doer is not important.

This is a question of aspect.

• отправить → perfective
• отправлять → imperfective

In the sentence Жалобу отправили вчера, the perfective verb отправили shows that the action is viewed as completed: the complaint got sent.

That is why it fits well with вчера when the speaker means a finished event.

Compare:

• Жалобу отправили вчера.
  They sent the complaint yesterday.
  The action is completed.

• Жалобу отправляли вчера.
  They were sending / tried to send / used the sending process yesterday.
  This sounds more process-oriented or context-dependent, and usually would need a special reason in context.

So отправили is the natural choice for a simple completed past event.

Отправили is the past tense plural form of отправить.

Russian past tense is formed from the infinitive stem plus endings showing gender/number:

• отправил = masculine singular
• отправила = feminine singular
• отправило = neuter singular
• отправили = plural

Notice that in the past tense, Russian does not show person the way English does. So отправили does not itself mean specifically you sent or they sent. It simply means sent with a plural subject, or in this kind of construction, an unspecified plural doer.

Yes, the word order is flexible, and starting with жалобу gives it a certain focus or topic status.

Russian word order is much freer than English because case endings show grammatical roles. So several orders are possible, for example:

• Жалобу отправили вчера.
• Вчера отправили жалобу.
• Вчера жалобу отправили.

These versions are all possible, but they are not identical in emphasis.

Жалобу отправили вчера often feels like:

• we are talking about the complaint, and then saying what happened to it
• or contrasting it with something else

For example:

• Письмо ещё не отправили, а жалобу отправили вчера.
  They haven’t sent the letter yet, but they sent the complaint yesterday.

So the sentence is grammatically normal, but the word order helps shape the information focus.

Вчера is an adverb meaning yesterday.

Because it is an adverb, it does not change for case, number, or gender.

That is different from nouns in time expressions. For example:

• в понедельник = on Monday
• утром = in the morning

Those involve noun forms. But вчера is just an adverb, so it stays the same.

In this sentence, вчера tells you when the action happened.

Yes. In practice, it can correspond to both, depending on how you translate it into natural English.

The Russian sentence literally uses an active plural verb, something like they sent. But because the subject is omitted and indefinite, English often translates it more naturally with a passive:

• They sent the complaint yesterday.
• The complaint was sent yesterday.

Which English version is best depends on context and style.

A useful rule is:

• if Russian has plural past tense with no subject, English may often use someone/they or a passive

The stress is:

• жа́лобу
• отпра́вили
• вчера́

A rough pronunciation guide:

• жа́лобу ≈ ZHA-luh-boo
• отпра́вили ≈ at-PRA-vee-lee
• вчера́ ≈ fchee-RA

A few notes:

• Unstressed о is often pronounced more like a in normal speech.
• вчера begins with a consonant cluster that may feel difficult at first.
• Stress matters a lot in Russian, so it is worth learning the stressed syllable together with each new word.

Then you would usually add the subject explicitly:

• Сотрудники отправили жалобу вчера.
  The employees sent the complaint yesterday.

• Мы отправили жалобу вчера.
  We sent the complaint yesterday.

• Они отправили жалобу вчера.
  They sent the complaint yesterday.

When the subject is included, отправили is still the same plural past form, but now the sentence no longer has that indefinite someone/they meaning.

Yes, absolutely. In Russian, this is a fully normal and complete sentence.

English learners sometimes expect every sentence to have an explicit subject, but Russian does not always require that. Here the verb form and the construction already give enough information:

• something happened
• it happened to жалобу
• it happened вчера
• the doer is unspecified

So this is not an incomplete sentence. It is a standard Russian way to express a completed action when the agent is not named.