Следовательница сказала, что виновный уже признался.

Следовательница is a feminine noun meaning female investigator.

A few useful points:

• следователь is the basic noun investigator
• следовательница specifically marks the person as female
• In real Russian, следователь is still often used for women too, especially in official or professional contexts
• следовательница can sound more conversational or more explicitly gender-marked

So this sentence makes the investigator clearly female.

Because the subject, следовательница, is feminine singular.

In the past tense, Russian verbs agree with gender and number:

• masculine: сказал
• feminine: сказала
• neuter: сказало
• plural: сказали

So:

• следовательница сказала = the female investigator said
• следователь сказал = the male investigator said

Because что introduces a subordinate clause.

The sentence has two parts:

• Следовательница сказала = the investigator said
• что виновный уже признался = that the culprit has already confessed / that the guilty person had already confessed

Russian normally puts a comma before что in this kind of reported-speech structure.

Here что means that.

It links the main clause to the content of what was said:

• Она сказала, что... = She said that...
• Я знаю, что... = I know that...
• Он думал, что... = He thought that...

In English, that is often optional, but in Russian что is normally stated.

Yes, виновный is originally an adjective meaning guilty. But here it is being used as a noun-like word, meaning the guilty person, the culprit, or the offender.

This is very common in Russian. An adjective can stand on its own if the noun is understood.

So:

• виновный человек = guilty person
• виновный = the guilty one / the culprit

In this sentence, виновный is masculine singular nominative.

Not exactly.

• виновный = guilty, culpable; often more formal or legal
• виноватый = at fault, guilty; often more conversational
• виновник = the person responsible for something, the cause of it

In a criminal or legal context, виновный fits very naturally.

So this sentence sounds more like legal/reporting language than casual conversation.

Признался is the past tense, masculine singular form of признаться.

Here признаться means:

• to confess
• to admit

The -ся is part of the verb. In this case, you should mostly learn признаться as a whole verb meaning to confess/admit, not try to translate -ся literally as oneself.

Compare:

• признать = to acknowledge, recognize, admit something
• признаться = to confess, admit

So виновный уже признался means the guilty person already confessed.

Because признался is perfective, and it presents the confession as a completed event.

• признался = confessed, admitted
• признавался = was confessing / used to confess / confessed repeatedly, depending on context

Since the sentence reports a result — the culprit has already confessed — the perfective form признался is the natural choice.

Russian does not backshift tenses in the same way English often does.

English may say:

• She said that the culprit had already confessed

Russian simply says:

• Следовательница сказала, что виновный уже признался.

The past tense признался is enough, and уже helps show that the confession happened earlier than the moment of speaking.

So this is a very normal Russian way to express what English often puts in the past perfect.

Уже means already.

It shows that the confession had happened before the relevant moment and is now an accomplished fact.

So:

• виновный признался = the culprit confessed
• виновный уже признался = the culprit has already confessed / had already confessed

In this sentence, уже makes the statement sound more final and informative.

Yes, but the original order is the most neutral:

• Следовательница сказала, что виновный уже признался.

Russian word order is flexible, but changes usually affect emphasis.

For example:

• Следовательница сказала, что уже признался виновный.
  This is more marked and sounds less neutral.

• Виновный уже признался, сказала следовательница.
  This puts more focus on the information about the culprit and can sound more narrative.

So the given version is probably the best basic model for learners.

Both the noun-like adjective and the verb would change:

• Следовательница сказала, что виновная уже призналась.

Changes:

• виновный → виновная
• признался → призналась

That is because Russian marks gender not only on adjectives, but also on past-tense verbs.