Нужно смазать петлю, чтобы дверь не скрипела.

Нужно is an impersonal word meaning it is necessary, it’s needed, or one needs to.

So Нужно смазать петлю literally means something like It is necessary to lubricate the hinge.

Russian often leaves out a general subject when English would use you, we, or someone in a vague way. The sentence is not saying exactly who must do it; it just states that the action is necessary.

You could make it more specific, for example:

• Мне нужно смазать петлю = I need to lubricate the hinge
• Тебе нужно смазать петлю = You need to lubricate the hinge

Because нужно is commonly followed by an infinitive in Russian.

The structure is:

• нужно + infinitive = need to / it is necessary to ...

So:

• Нужно смазать = need to lubricate

This is a very common pattern:

• Нужно идти = We need to go / It’s necessary to go
• Нужно подождать = We need to wait

Смазать is perfective, while смазывать is imperfective.

Here, the speaker is talking about one complete action with a desired result: lubricate the hinge so the door stops squeaking. That is why смазать fits best.

• смазать = lubricate it once, successfully, as a completed action
• смазывать = lubricate in general, repeatedly, or as a process

Compare:

• Нужно смазать петлю = The hinge needs to be lubricated / We need to lubricate the hinge
• Нужно смазывать петли регулярно = The hinges need to be lubricated regularly

Yes, петля can mean loop, noose, or hinge, depending on context.

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

So:

• дверная петля = door hinge
• смазать петлю = lubricate the hinge

Russian often uses words with several related meanings, and context tells you which one is intended.

Because петлю is the accusative singular form of петля.

The verb смазать takes a direct object, and the direct object goes in the accusative case.

• nominative: петля = hinge
• accusative: петлю = hinge as the object of the action

So:

• Петля скрипит = The hinge is squeaking
  here петля is the subject
• Нужно смазать петлю = It’s necessary to lubricate the hinge
  here петлю is the object

Yes, in real life a door often has several hinges, but Russian can still use the singular if the speaker is thinking of the problematic hinge or speaking in a simple, practical way.

So смазать петлю can mean:

• lubricate the hinge that is causing the squeak, or
• lubricate the hinge as a generic object in this situation

If the speaker definitely meant several hinges, they could say:

• Нужно смазать петли = The hinges need to be lubricated

Here чтобы means so that, in order that, or more naturally in English, so.

It introduces a purpose clause: the reason for doing the first action.

So the sentence structure is:

• Нужно смазать петлю = the action
• чтобы дверь не скрипела = the purpose/result wanted

Together:

• Need to lubricate the hinge so that the door doesn’t squeak

Because the second part has its own subject: дверь.

If the same person does both actions, Russian often uses an infinitive after чтобы or even no чтобы at all, depending on the sentence. But here, the first action is someone lubricates, while the second action is the door doesn’t squeak. Since the second part has its own subject, Russian uses a full clause:

• чтобы дверь не скрипела

That is very natural Russian.

That is a very common and important Russian pattern.

After чтобы, Russian often uses a form that looks like the past tense, even when the meaning is about a desired present or future result.

So:

• чтобы дверь не скрипела means so that the door wouldn’t squeak / so that the door doesn’t squeak

It is not talking about the door squeaking in the past. It is expressing the intended outcome.

This is normal after чтобы:

• Я закрыл окно, чтобы не было холодно = I closed the window so that it wouldn’t be cold
• Говори громче, чтобы все слышали = Speak louder so that everyone can hear

Because скрипела agrees with дверь.

In Russian, past-tense verb forms show gender and number:

• masculine: скрипел
• feminine: скрипела
• neuter: скрипело
• plural: скрипели

Since дверь is a feminine singular noun, the verb is:

• дверь не скрипела

Even though the whole clause expresses purpose rather than simple past time, the form still agrees with the noun in this way.

Because чтобы introduces a subordinate clause.

Russian punctuation normally puts a comma before subordinate-clause markers like что, когда, потому что, если, чтобы, etc.

So:

• Нужно смазать петлю, чтобы дверь не скрипела.

That comma is required in standard written Russian.

Yes. Надо and нужно are often very similar here.

• Нужно смазать петлю = It’s necessary to lubricate the hinge
• Надо смазать петлю = Need to lubricate the hinge

Very roughly:

• надо can sound a bit more conversational
• нужно can sound slightly more neutral or formal

In everyday speech, both are common and natural.