Иглу нужно заменить.

Because иглу is the accusative singular form of игла.

Here, the needle is the direct object of заменить:

• игла = nominative, “needle”
• иглу = accusative, “needle” as the thing being replaced

So the structure is basically: [The needle] needs [to be replaced], but in Russian the object appears directly in the accusative:

• Иглу нужно заменить.

Нужно means it is necessary, it is needed, or more naturally here, needs to.

It is often used in impersonal sentences, where there is no clear grammatical subject:

• Нужно работать. = “It is necessary to work” / “One must work.”
• Иглу нужно заменить. = “The needle needs to be replaced.”

So нужно is not agreeing with иглу. It is not describing the needle like an adjective would. It is part of an impersonal construction meaning necessary / must / need to.

Because after нужно, Russian commonly uses an infinitive to say what needs to be done.

Pattern:

• нужно + infinitive

Examples:

• Нужно уйти. = “It is necessary to leave.”
• Нужно купить хлеб. = “We need to buy bread.”
• Иглу нужно заменить. = “The needle needs to be replaced.”

So заменить is the action that is necessary.

This is about aspect.

• заменить = perfective: to replace, as a completed action
• заменять = imperfective: to be replacing / to replace in general / repeatedly

In Иглу нужно заменить, the idea is that the needle should be replaced once, as a completed result. That is why заменить sounds natural.

Compare:

• Иглу нужно заменить. = “The needle needs to be replaced.”
• Иглу нужно заменять регулярно. = “The needle needs to be replaced regularly.”

The second one uses imperfective because it refers to a repeated/general process.

In meaning, it is close to an English passive: “The needle needs to be replaced.”

But grammatically, Russian is doing it differently. It is not a normal passive form here. Instead, it uses:

• the object in the accusative: иглу
• the impersonal word нужно
• the infinitive заменить

So literally, the structure is closer to:

• “The needle — it is necessary to replace.”

This is a very common Russian way to express ideas that English often puts in the passive.

The sentence does not specify who must do it.

That is normal in Russian. The sentence simply states that replacing the needle is necessary.

If you want to add who needs to do it, you can:

• Мне нужно заменить иглу. = “I need to replace the needle.”
• Тебе нужно заменить иглу. = “You need to replace the needle.”
• Нам нужно заменить иглу. = “We need to replace the needle.”

Without that dative pronoun, the sentence stays general and impersonal:

• Иглу нужно заменить. = “The needle needs to be replaced.”

Yes. Russian word order is flexible.

These are all possible, with slightly different emphasis:

• Иглу нужно заменить.
• Нужно заменить иглу.

The first version highlights иглу a bit more, as the thing under discussion.
The second sounds a bit more neutral as “It’s necessary to replace the needle.”

In many contexts, both are natural.

Formally, иглу can also be the indeclinable noun meaning igloo. But in this sentence, the meaning is normally clear from context.

Two possibilities:

• иглу from игла = “needle” (accusative singular)
• иглу = “igloo” (borrowed noun, usually indeclinable)

So:

• Иглу нужно заменить.

Most likely means:

• “The needle needs to be replaced.”

But in some unusual context, it could theoretically mean:

• “The igloo needs to be replaced.”

Context tells you which one is meant.

No, that is not correct.

Why not:

1. игла would be nominative, but here you need the accusative: иглу
2. нужна is the feminine short form of нужный, and that is a different structure

Correct:

• Иглу нужно заменить.

You can use нужен / нужна / нужно / нужны in other patterns, but not like this one. For example:

• Мне нужна игла. = “I need a needle.”

That means “a needle is needed by me”, not “the needle needs to be replaced.”

So don’t confuse:

• Мне нужна игла. = “I need a needle.”
• Иглу нужно заменить. = “The needle needs to be replaced.”

You keep нужно and change the time through context or the surrounding words, or you use a fuller phrasing.

Common examples:

• Иглу нужно было заменить. = “The needle needed to be replaced.”
• Иглу нужно будет заменить. = “The needle will need to be replaced.”

So:

• нужно = present/general necessity
• нужно было = past necessity
• нужно будет = future necessity

This is a very useful pattern in Russian.