То, что сказала свидетельница, изменило всё расследование.

This is a very common Russian pattern: то, что...

• то is a pointing word: that / that thing
• что сказала свидетельница is the clause that explains what that thing is

So то, что сказала свидетельница means what the witness said or the thing that the witness said.

Russian often uses this two-part structure where English can use a single word like what. A version without то is possible in some contexts, but то, что... is the normal, clear choice here.

Because что сказала свидетельница is a subordinate clause.

The structure is:

• То — the pointing word
• что сказала свидетельница — the subordinate clause
• изменило всё расследование — the main clause

So the commas mark where the subordinate clause begins and ends:

• the first comma introduces the что-clause
• the second comma closes it before the main predicate изменило

Russian uses commas with subordinate clauses much more consistently than English does.

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

Here, the subject of сказала is свидетельница:

• свидетельница = feminine singular
• so the verb is сказала

Compare:

• свидетель сказал — a male witness said
• свидетельница сказала — a female witness said
• свидетели сказали — witnesses said

It is in the nominative singular.

That is because свидетельница is the subject of the verb сказала inside the subordinate clause.

So inside что сказала свидетельница, the basic relationship is:

• свидетельница — subject
• сказала — verb

Because the subject of the main verb is not свидетельница.

The subject of изменило is the whole phrase:

То, что сказала свидетельница

Russian often treats this kind of clause-based subject as neuter singular, especially because it is headed by то, which is neuter.

So:

• то → neuter singular
• therefore изменило → neuter singular past form

That is why the sentence does not use изменила.

Because расследование is neuter singular.

So the word meaning whole / entire must also be neuter singular:

• всё расследование = the whole investigation

Compare:

• всё расследование — neuter singular
• все расследования — plural, all investigations

Also, the ё in всё is correct here. In many Russian texts, ё is often written as е, but the meaning from context is still всё.

It is in the accusative singular, because it is the direct object of изменило.

The verb изменить takes a direct object:

• изменить что? — to change what?

Answer:

• всё расследование

For an inanimate neuter noun, the accusative singular looks the same as the nominative singular, so расследование does not change its form.

Because the sentence presents both actions as completed events.

• сказала suggests the witness made a statement
• изменило suggests that statement changed the investigation as a result

This creates a clear cause-and-effect sequence.

If you used imperfective verbs, the meaning would shift toward process, repetition, or background description. For example:

• говорила would sound more like was speaking / used to say
• изменяло would suggest repeated or ongoing changing

That would not fit this sentence as well.

Yes, Russian word order is flexible, but the original order is the most natural and neutral.

Starting with То, что сказала свидетельница puts focus on that whole idea as the subject of the sentence.

A different order is possible for emphasis, for example:

Изменило всё расследование то, что сказала свидетельница.

This still means the same thing, but it sounds more marked or dramatic.

So the original version is usually the best one for normal statement style.

Yes.

• свидетель = witness, masculine
• свидетельница = female witness

In this sentence, свидетельница tells you that the witness is female, and that is why the verb in the subordinate clause is сказала.

The feminine form is completely normal in modern Russian.