Этот монолог был таким тихим, что даже в последнем ряду все молчали.

Because монолог is a masculine singular noun in Russian.

• этот = masculine singular
• эта = feminine singular
• это = neuter singular

So этот монолог means this monologue.

Because монолог is the subject of the sentence — it is the thing being described.

The basic structure is:

• Этот монолог = the subject
• был таким тихим = was so quiet

Subjects normally appear in the nominative case in Russian.

Был is the past-tense form of быть (to be).

Russian usually omits to be in the present tense:

• Монолог тихий = The monologue is quiet.

But in the past tense, Russian normally uses the past form:

• Монолог был тихим = The monologue was quiet.

So был is necessary because the sentence is in the past.

After быть in the past tense, Russian often uses the instrumental case for a predicate noun or adjective.

So:

• тихий → nominative
• тихим → instrumental

And because таким modifies тихим, it must match it:

• masculine
• singular
• instrumental

That gives таким тихим.

This is very common after был / была / было / были:

• Он был уставшим = He was tired.
• Она была счастливой = She was happy.

This is a very common Russian structure meaning so ... that ...

In this sentence:

• таким тихим = so quiet
• что = that

So the pattern is:

• Он был таким уставшим, что сразу уснул. = He was so tired that he fell asleep immediately.

Here:

• Этот монолог был таким тихим, что... = This monologue was so quiet that...

Not in this sentence as written.

Тихим is an adjective describing the noun монолог.
So Russian uses an adjective form:

• монолог был тихим = the monologue was quiet

Тихо is an adverb or a predicative word, and it usually describes how something happens or the general environment:

• Он говорил тихо = He spoke quietly.
• В комнате было тихо = It was quiet in the room.

So:

• монолог был тихим = the monologue itself was quiet
• он говорил тихо = he spoke quietly

Because в here means in in the sense of location, and after в with location Russian uses the prepositional case.

• ряд = row
• в ряду = in the row
• в последнем ряду = in the last row

The adjective последнем also has to agree with ряду in case, number, and gender.

Даже means even.

In this sentence it emphasizes в последнем ряду:

• даже в последнем ряду = even in the last row

The idea is that the result was noticeable all the way to the back.
Russian often places даже right before the word or phrase it emphasizes.

Compare:

• Даже он понял. = Even he understood.
• Он даже понял. = He even understood.

Because все here means everyone / all the people, so the verb is plural.

• все молчали = everyone was silent / everyone kept quiet

Even though English everyone is grammatically singular, Russian все in this meaning normally takes a plural verb.

So:

• Все пришли. = Everyone came.
• Все молчали. = Everyone was silent.

Молчали describes a state: they were silent or kept quiet.

Замолчали means fell silent or became silent, focusing on the moment the silence began.

So the difference is:

• все молчали = everyone was silent
• все замолчали = everyone fell silent

In this sentence, the result clause describes the continuing situation, so молчали fits better.

Not completely. Russian word order is flexible, but the chosen order sounds natural and highlights the result nicely.

Current order:

• Этот монолог был таким тихим, что даже в последнем ряду все молчали.

This presents:

1. the subject
2. its quality
3. the result

You could move things around for emphasis, for example:

• Даже в последнем ряду все молчали.
• Все даже в последнем ряду молчали.

But each version changes the focus slightly. The original is a normal, natural literary order.

You may hear forms like такой тихий, especially in colloquial speech, but in a sentence with был the form таким тихим is the more standard and grammatically expected version here.

So for learners, the safest model is:

• Он был таким уставшим, что...
• Она была такой доброй, что...
• Этот монолог был таким тихим, что...

The key point is that after был / была / было / были, Russian very often uses the instrumental in this kind of description.