Следовательница поняла, что эта подозреваемая невиновна.

Because Russian past-tense verbs agree with the subject in gender and number.

Here the subject is следовательница, which is feminine singular, so the verb is поняла.

Compare:

• следователь понял = the male investigator understood
• следовательница поняла = the female investigator understood
• следователи поняли = the investigators understood

Поняла is the perfective past tense. It presents the understanding as a completed event, often meaning realized or came to understand.

Понимала is imperfective. It usually means understood in an ongoing or general sense, not focusing on the moment of realization.

So:

• поняла = she realized / she understood at that moment
• понимала = she understood / she was aware

In this sentence, the idea is that the investigator reached a conclusion, so поняла fits better.

Следовательница means female investigator. It is a feminine noun.

Yes, you can also encounter следователь, which is the masculine form and is often used as a general job title regardless of gender in some contexts. The choice between следователь and следовательница can depend on style, preference, and how explicitly the speaker wants to mark the person as female.

So:

• следователь = investigator
• следовательница = female investigator

Here что means that and introduces a subordinate clause:

Следовательница поняла, что эта подозреваемая невиновна.
= The investigator understood that this suspect was innocent.

Russian normally uses a comma before что when it introduces a subordinate clause. This is standard punctuation.

So the sentence has:

• main clause: Следовательница поняла
• subordinate clause: что эта подозреваемая невиновна

Literally, эта means this and agrees with a feminine singular noun.

Russian has no articles, so English the and a are often not translated by any separate word. Because of that, эта can sometimes sound more specific than English the, something like this particular or this one.

So:

• эта подозреваемая = this female suspect / this particular suspect

If you removed эта, the sentence would still be grammatical:

• Следовательница поняла, что подозреваемая невиновна.

That would be less specifically demonstrative.

Because it originally is an adjectival form, but in Russian adjectives and participles can be used as nouns when the noun they describe is understood from context.

Подозреваемая literally means something like the suspected one in feminine form, but in normal usage it means female suspect.

This is very common in Russian. English does something similar sometimes with expressions like the accused or the wounded.

Here:

• эта подозреваемая = this female suspect

A very common question. The direct object of поняла is not the noun подозреваемая by itself. What the investigator understood is the whole idea expressed by the clause:

что эта подозреваемая невиновна
= that this suspect is innocent

Inside that subordinate clause, эта подозреваемая is the subject of невиновна, so it stays in the nominative case.

So the structure is:

• Следовательница поняла = The investigator understood
• что эта подозреваемая невиновна = that this suspect is innocent

If подозреваемую were accusative, it would suggest a different structure.

They are quite different:

• подозреваемая = a female suspect, someone suspected of a crime
• подозрительная = suspicious, shady, making people suspicious

So:

• эта подозреваемая = this female suspect
• эта женщина подозрительная = this woman is suspicious

English learners often mix these up because both are related to suspicion, but only подозреваемая means a legal or investigative suspect.

Невиновна is the short form of the adjective невиновный and is commonly used in predicate position, after an understood is.

So:

• эта подозреваемая невиновна = this suspect is innocent

The full form невиновная is usually used attributively, meaning it directly modifies a noun:

• невиновная женщина = an innocent woman

In this sentence, the adjective is not describing the noun inside a noun phrase; it is making a statement about the subject. That is why the short form невиновна is natural.

Also notice agreement:

• masculine: невиновен
• feminine: невиновна
• neuter: невиновно
• plural: невиновны

Yes, in a way. In the present tense, Russian normally omits the verb to be.

So:

• эта подозреваемая невиновна literally looks like this suspect innocent
• but it means this suspect is innocent

This is completely normal in Russian.

If you wanted past or future, you would use forms of быть:

• эта подозреваемая была невиновна = this suspect was innocent
• эта подозреваемая будет невиновна = this suspect will be innocent

Эта is nominative feminine singular.

It agrees with подозреваемая, which is also nominative feminine singular.

Russian demonstratives change form to match the noun they go with:

• masculine nominative: этот
• feminine nominative: эта
• neuter nominative: это
• plural nominative: эти

Since подозреваемая is feminine singular, the correct form is эта.

Yes. Russian word order is more flexible than English because endings show grammatical roles.

The given order is fairly neutral:

• Следовательница поняла, что эта подозреваемая невиновна.

But other orders are possible for emphasis. For example:

• Следовательница поняла, что невиновна эта подозреваемая.
  This can emphasize that it is this suspect who is innocent.
• Что эта подозреваемая невиновна, следовательница поняла не сразу.
  This emphasizes the whole subordinate clause.

Even though word order can change, not every order sounds equally natural in every context. The original sentence is a normal, clear version.