Собственник подписал протокол.

Because it is past tense, masculine, singular.

In Russian past tense, the verb agrees with the subject in gender and number:

• подписал = he signed / a masculine subject signed
• подписала = she signed / a feminine subject signed
• подписало = it signed / a neuter subject signed
• подписали = they signed / plural subject signed

Since собственник is a masculine singular noun, the verb is подписал.

It is in the nominative case because it is the subject of the sentence — the person who performed the action.

So in:

• Собственник подписал протокол.

собственник = the owner / proprietor
This is the one who did the signing.

It is in the accusative, but for many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: протокол
• accusative: протокол

That is why the form does not change.

This is very common in Russian:

• Я вижу стол.
• Он подписал протокол.

Both стол and протокол are masculine inanimate nouns, so nominative and accusative are the same.

Russian has no articles.

So Собственник подписал протокол can mean things like:

• The owner signed the protocol
• An owner signed the protocol
• The proprietor signed a report

Which one is meant depends on context.

Russian usually leaves this information to the situation, previous sentences, or general context rather than using articles.

Собственник means owner, especially in a fairly formal, legal, or official sense.

It often refers to:

• the owner of property
• the legal owner of something
• a person who has ownership rights

Compared with some other words:

• владелец = owner / holder / possessor, often broader in use
• хозяин = owner, master, host; often more everyday and less legal/formal

So собственник sounds more official than a casual everyday word in many contexts.

Протокол is often translated as protocol, but in many Russian contexts it means a formal written record such as:

• minutes of a meeting
• an official report
• a record of proceedings
• sometimes a police report or other official document

So the best English translation depends on context.
In a business or administrative setting, minutes or official record may sound more natural than the English word protocol.

Подписал is the perfective past form of подписать. It shows a completed action:

• подписал = signed, completed the signing

This implies the result matters: the document ended up signed.

Compare:

• подписал = signed it, finished signing it
• подписывал = was signing / used to sign / signed at some point without focusing on completion

So this sentence presents the signing as a finished event.

Yes, but the meaning would change.

• Собственник подписал протокол.
  = The owner signed the protocol.
  Focus: the action was completed.

• Собственник подписывал протокол.
  = The owner was signing the protocol / signed the protocol in some context.
  Focus: the process, background, repetition, or an action viewed without emphasizing completion.

In most simple statements about a completed result, подписал is the more natural choice.

Yes. Russian word order is relatively flexible, though the meaning emphasis changes.

Basic neutral order:

• Собственник подписал протокол.

Other possible orders:

• Протокол подписал собственник.
• Подписал протокол собственник.

These usually sound more marked and may emphasize:

• who signed it
• what was signed
• contrast with something previously mentioned

So the original order is the most neutral and straightforward.

The main word stress is:

• со́бственник
• подписа́л
• протоко́л

So the full sentence is:

• Со́бственник подписа́л протоко́л.

A rough pronunciation guide:

• SOB-stven-neek pad-pee-SAL prah-ta-KOL

A few notes:

• Unstressed о is often pronounced more like a in normal speech.
• Russian п is unaspirated, so it is softer than English p in words like pin.
• Stress is important in Russian and can change from word to word unpredictably, so it is worth learning each word with its stress.

Yes — it is a very typical simple Russian sentence structure:

• subject: собственник
• verb: подписал
• direct object: протокол

So it follows a basic Subject–Verb–Object pattern, which is often the most neutral way to present information in Russian.

It is a good example of:

• nominative subject
• past-tense verb agreeing with the subject’s gender/number
• accusative direct object
• perfective aspect for a completed action