Сначала нужно проверить соединение.

Нужно is an impersonal predicative word meaning something like it is necessary or one needs to.

So:

• нужно проверить = it is necessary to check / you need to check

This kind of sentence does not name a specific subject such as я, ты, or мы. In English, we often translate it with you need to..., but in Russian it is more neutral and impersonal.

This is a very common pattern:

• Нужно подождать. = You need to wait.
• Нужно позвонить. = It is necessary to call.

After нужно, Russian normally uses the infinitive to say what action is necessary.

Structure:

• нужно + infinitive

So here:

• нужно проверить = need to check

Other examples:

• Нужно читать. = You need to read.
• Нужно сделать это. = You need to do this.

This is about aspect.

• проверить = perfective
• проверять = imperfective

In this sentence, проверить is used because it refers to checking the connection as a complete action, usually once, with the focus on getting the result.

So:

• Сначала нужно проверить соединение. = first, you need to check the connection and finish that step.

If you used проверять, it would sound more like a process, repetition, or general activity:

• Нужно проверять соединение регулярно. = You need to check the connection regularly.

So in your sentence, проверить is the natural choice because it is a single completed step in a sequence.

Соединение is in the accusative case, because it is the direct object of проверить.

You check what?

• проверить соединение

However, соединение is a neuter inanimate noun, and for nouns like this, the accusative singular is identical to the nominative singular.

So:

• nominative: соединение
• accusative: соединение

That is why the form does not visibly change.

Сначала means first, at first, or to begin with, depending on context.

In this sentence, it introduces the first step in a sequence:

• Сначала нужно проверить соединение.
• First, you need to check the connection.

It often suggests that more steps will follow.

For example:

• Сначала нужно проверить соединение, потом перезагрузить роутер.
• First you need to check the connection, then restart the router.

Yes, but they are often very close in meaning.

• сначала = first / at first / in the beginning
• сперва = also first, often a bit more conversational in many contexts

In many sentences, they can replace each other:

• Сначала нужно проверить соединение.
• Сперва нужно проверить соединение.

Both are natural.
For a learner, the safest choice is that сначала is extremely common and neutral.

Russian often leaves the subject unstated when it is:

• obvious from context
• not important
• meant to be general

Here, the idea is not you personally, but rather this is the necessary step.

So instead of saying:

• Ты должен проверить соединение. = You must check the connection.

Russian uses the softer, more neutral:

• Нужно проверить соединение. = You need to check the connection.

This is especially common in instructions, manuals, troubleshooting, and general advice.

Yes. Russian word order is flexible, though the most natural order depends on emphasis.

The original:

• Сначала нужно проверить соединение.

This is a very natural neutral sentence: first, the listener is told that the first step is checking the connection.

Other possible orders:

• Нужно сначала проверить соединение.
• Проверить соединение нужно сначала.

These can all be grammatical, but they shift emphasis slightly.

Very roughly:

• Сначала нужно проверить соединение. → emphasizes first
• Нужно сначала проверить соединение. → also natural, slightly more centered on the action
• Проверить соединение нужно сначала. → stronger emphasis on checking the connection as the first thing

For learners, the original version is a very good default.

The stress is:

• снача́ла
• ну́жно
• прове́рить
• соедине́ние

Stress matters in Russian pronunciation, so it is worth learning these with the stress included.

A rough pronunciation guide:

• снача́ла → snuh-CHA-luh
• ну́жно → NOOZH-nuh
• прове́рить → pruh-VYE-ritʹ
• соедине́ние → suh-ee-dee-NYE-nee-ye

Usually, yes, in this kind of context.

Соединение is a general word meaning connection, link, or joining. In technical contexts, it often means:

• internet connection
• network connection
• device connection

So in a troubleshooting sentence like this, проверить соединение usually means:

• check the network connection
• check whether the device is connected properly

A related word is связь, but соединение is often more natural for technical or system-level connections.

For example:

• Интернет-соединение = internet connection
• сетевое соединение = network connection

Yes. Надо and нужно are often very similar.

• нужно = need to / it is necessary
• надо = need to / have to

In many everyday contexts, they are interchangeable:

• Сначала нужно проверить соединение.
• Сначала надо проверить соединение.

Both are natural.

A very rough tendency:

• нужно can sound a little more neutral or formal
• надо can sound a little more conversational

But in many real situations, the difference is small.