Мастер смазал петлю, и дверь перестала скрипеть.

Because петлю is the accusative singular form of петля.

• Dictionary form: петля = hinge / loop
• In the sentence, it is the direct object of смазал (lubricated / oiled), so it changes case.
• For many feminine nouns ending in -я, the accusative singular ends in -ю.

So:

• петля = a hinge
• смазал петлю = lubricated the hinge

Not exactly. In this sentence, мастер most naturally means something like:

• repairman
• handyman
• craftsman
• technician

It usually refers to someone skilled at practical work. So here it is not master in the sense of boss or expert in a grand abstract sense, but rather a person who came and fixed something.

Смазал is:

• past tense
• masculine singular
• perfective

It comes from the verb смазать.

What that means here:

1. Past tense: the action happened in the past.
2. Masculine singular: the subject, мастер, is masculine.
3. Perfective: the action is seen as completed.

So мастер смазал петлю means the handyman finished lubricating the hinge.

If you used an imperfective form like мазал, it would sound more like:

• he was lubricating it
• he used to lubricate it
• he was in the process of doing it

But here the sentence is about a completed fix, so смазал is the natural choice.

Because дверь is a feminine noun.

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

• перестал = masculine
• перестала = feminine
• перестало = neuter
• перестали = plural

Since дверь is feminine, we get:

• дверь перестала = the door stopped

Because перестать means to stop doing something, and it is normally followed by an infinitive.

So the structure is:

• перестать + infinitive

Examples:

• перестать говорить = to stop talking
• перестать работать = to stop working
• перестать скрипеть = to stop squeaking/creaking

In your sentence:

• дверь перестала скрипеть = the door stopped squeaking

That is a very common pattern in Russian.

• перестала is perfective because it marks a completed change: the door came to a stop.
• скрипеть is imperfective because it names the ongoing activity/state that stopped.

So the idea is:

• the stopping is a completed event
• the squeaking is the activity that used to be happening

This is very natural Russian:

• перестал курить = stopped smoking
• перестала плакать = stopped crying
• перестали спорить = stopped arguing

Because this sentence joins two full clauses, each with its own subject and verb:

1. Мастер смазал петлю
2. дверь перестала скрипеть

Russian normally uses a comma before и when it connects two independent clauses like that.

So the comma is needed because this is essentially:

• The handyman lubricated the hinge, and the door stopped squeaking.

If и joined only two words, not two clauses, there would usually be no comma.

Because дверь is the subject of the second clause:

• дверь перестала скрипеть = the door stopped squeaking

Subjects are normally in the nominative case in Russian.

So:

• дверь = nominative singular
• it is the thing performing the action перестала

Yes, петля can mean several related things depending on context, including:

• loop
• noose
• hinge

In this sentence, because we are talking about a door squeaking, петля clearly means door hinge.

Context is what tells you which meaning is intended.

No, it does not have to. Смазать already means to lubricate / grease / oil, so the substance can be left unstated if it is not important.

Russian often leaves out information that is obvious from context.

So:

• Мастер смазал петлю = The handyman lubricated the hinge.

If you wanted to mention the substance, you could:

• смазал петлю маслом = lubricated the hinge with oil

But it is completely natural not to mention it.

It is basically neutral, natural narrative word order.

• Мастер смазал петлю = first the person, then the completed action, then the object
• и дверь перестала скрипеть = and then the result

Russian word order is flexible, but changes in order often affect emphasis.

For example:

• Петлю смазал мастер would put more focus on the hinge
• Скрипеть дверь перестала would emphasize the stopping of the squeak

Your original sentence sounds like a straightforward report of what happened.

Yes, in some contexts it could.

• смазал петлю = lubricated the hinge
• смазал петли = lubricated the hinges

The singular suggests:

• one hinge was the problem, or
• Russian is focusing on the relevant hinge rather than all of them

A door often has more than one hinge, but singular is still perfectly natural if one squeaky hinge is meant.

The main stresses are:

• мАстер
• смазАл
• пЕтлю
• двЕрь
• перестАла
• скрипЕть

So roughly:

мАстер смазАл пЕтлю, и двЕрь перестАла скрипЕть

Stress is very important in Russian, so it is worth learning words together with their stressed syllables.