Потом появилась свидетельница, которая видела, как всё началось.

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

The subject here is свидетельница, which is feminine singular, so the verb must also be feminine singular:

• появился = masculine singular
• появилась = feminine singular
• появилось = neuter singular
• появились = plural

So появилась свидетельница is the correct match.

Свидетельница means female witness. It is the feminine form corresponding to свидетель.

So:

• свидетель = witness, male witness, or sometimes witness in a general sense
• свидетельница = female witness

Since the sentence is specifically talking about a woman, Russian uses свидетельница.

Которая is a relative pronoun meaning who, which, or that, depending on the English translation.

Here it introduces a relative clause:

• свидетельница, которая видела... = the witness who saw...

Russian normally keeps this pronoun. Unlike English, you usually cannot just omit it.

Because который changes form according to gender, number, and case.

Here it refers back to свидетельница, which is feminine singular, so the gender/number part must be feminine singular: которая.

It is also in the nominative case because it is the subject of видела:

• которая видела = who saw

If it had a different role in the clause, the form would change. For example:

• свидетельница, которую мы нашли = the witness whom we found
• свидетельница, с которой я говорил = the witness with whom I spoke

For the same reason as появилась: past-tense verbs agree in gender and number.

The person doing the seeing is которая, which refers to свидетельница, a feminine singular noun. So the verb is feminine singular:

• которая видела = who saw

If it were a man, it would be который видел.

This is an aspect question.

• видела is from видеть, an imperfective verb
• увидела is from увидеть, a perfective verb

In this sentence, видела, как всё началось means she saw/witnessed how it began. The focus is on the fact that she was a witness to the event.

If you said увидела, the nuance would shift more toward caught sight of or managed to see the moment.

So видела is very natural when describing what someone witnessed.

Here как means something like how or the way.

So:

• видела, как всё началось = saw how it all began

After verbs of perception such as видеть, слышать, and sometimes заметить, Russian often uses как to introduce what was observed.

It does not necessarily mean the person understood the mechanism of what happened. It can simply mean they witnessed the event unfold.

Because the subject is всё, and всё is grammatically treated as neuter singular in this kind of sentence.

So the verb must also be neuter singular:

• всё началось = it all began

This is very common in Russian:

• всё кончилось
• всё изменилось
• всё случилось

Even though всё refers to many things in a broad sense, grammatically it behaves as a singular neuter word here.

Again, this is about aspect.

• началось is perfective
• начиналось is imperfective

Началось focuses on the single completed starting point: it began.

That fits the sentence well, because the witness saw the moment when everything started.

Начиналось would suggest more of a process, background description, or repeated beginning, which is not the main idea here.

Because Russian uses commas to separate subordinate clauses.

In this sentence there are two such clauses:

1. которая видела, как всё началось
   This is a relative clause describing свидетельница.

2. как всё началось
   This is a subordinate clause inside the first clause, explaining what she saw.

So the structure is:

• Потом появилась свидетельница, которая видела, как всё началось.

Russian punctuation is generally more consistent than English here: subordinate clauses are usually marked with commas.

Потом means then, afterwards, or later.

Putting it first is very natural in narrative Russian. It sets the sequence of events:

• first something happened
• потом this witness appeared

Starting with Потом gives the sentence a storytelling feel. Russian often places time words early in the sentence to organize the flow of events.

Russian word order is flexible, and different orders create different emphasis.

Потом появилась свидетельница is very natural in narration because it presents the witness as a newly entering figure in the story. The verb comes before the noun almost like Then a witness appeared / showed up.

If you say Потом свидетельница появилась, it is still possible, but it sounds more marked and puts more emphasis on свидетельница itself.

So the original order is a smooth, idiomatic storytelling order.

Because the word here is всё, meaning everything or it all.

The letter ё shows both the pronunciation and the correct word. This helps distinguish it from все, which usually means everyone or all in the plural.

So:

• всё = everything, it all
• все = everyone, all people, all things in plural contexts

In many Russian texts, ё is often replaced in writing by е, but learners should know that the correct underlying form here is всё.