Сначала нужно подставить ведро.

Сначала means first, at first, or to begin with.

In this sentence, it tells you that this is the first step in a process:

• Сначала нужно подставить ведро. = First, you need to place a bucket underneath.

It is an adverb, so it describes when the action should happen in relation to other actions.

Нужно means it is necessary, one must, or you need to.

Russian often uses this kind of impersonal construction, where there is no explicit subject like I, you, or we.

So:

• нужно подставить ведро = it is necessary to place the bucket
• in more natural English: you need to place the bucket

This structure is very common in instructions and explanations.

Because Russian often leaves out the subject when it is general or obvious from context.

In English, instructions often use you need to...
In Russian, it is very natural to say simply:

• нужно подставить ведро

This does not sound incomplete. It is a standard way to give instructions, like:

• Нужно подождать. = You need to wait.
• Нужно нажать кнопку. = You need to press the button.

Here подставить means something like:

• to place underneath
• to put under
• to position under something in order to catch or receive something

So if a bucket is involved, the idea is usually:

• put the bucket under a leak
• put the bucket under a pipe
• put the bucket where something will drip or pour into it

The verb is made from ставить (to put / to place standing) with the prefix под-, which often adds the idea of under.

This is about aspect.

• подставить = perfective
• подставлять = imperfective

In this sentence, подставить is used because the speaker is talking about a single complete action:

• first, place the bucket underneath

In instructions, Russian often uses the perfective infinitive when the focus is on completing a step.

Compare:

• Нужно подставить ведро. = You need to place the bucket underneath.
  (one completed action)

• Нужно подставлять ведро каждый раз. = You need to place the bucket underneath every time.
  (repeated/habitual action)

Ведро is the noun bucket.

Here it is the direct object of подставить, so it is in the accusative case.

However, for neuter inanimate nouns, the accusative form is often the same as the nominative form. So:

• nominative: ведро
• accusative: ведро

That is why the word does not change.

A very common clue is the ending -о.

Many Russian nouns ending in -о are neuter, including:

• ведро = bucket
• окно = window
• письмо = letter

So ведро is a neuter noun, and that affects agreement and case patterns.

Yes. Russian word order is fairly flexible, although some orders sound more natural in certain contexts.

The neutral version here is:

• Сначала нужно подставить ведро.

But you could also hear:

• Ведро сначала нужно подставить.
• Нужно сначала подставить ведро.

These alternatives change the emphasis a little, not the core meaning.

• starting with сначала emphasizes the sequence
• starting with ведро emphasizes the bucket
• starting with нужно emphasizes necessity

Yes, very possibly. Russian often leaves out details that are clear from context.

For example, the full idea might be:

• place the bucket under the leak
• place the bucket under the tap
• place the bucket under the pipe

But if everyone already knows what the bucket goes under, Russian can simply say:

• Сначала нужно подставить ведро.

So the sentence may sound slightly incomplete by itself in English, but in Russian it is perfectly normal if the context is clear.

A helpful stress pattern is:

• снача́ла ну́жно подста́вить ведро́

Approximate pronunciation:

• sna-CHA-la NOOZH-na pad-STA-vit' vee-DRO

A few notes:

• ж in нужно sounds like the s in measure
• the final -ть in подставить is soft
• the stress in ведро́ is on the last syllable

Yes, it sounds natural, especially in:

• instructions
• practical advice
• explanations of a process

It has a very typical Russian instructional pattern:

• Сначала = first
• нужно = it is necessary / you need to
• [perfective infinitive] = complete this step

So structurally it is a very useful model to learn.