Раз ты уже дома, помоги мне донести коробку до кухни.

Here раз does not mean one time / once. It is a conjunction meaning something like:

• since
• seeing that
• given that

So Раз ты уже дома... means Since you’re already home...

This use of раз is common in spoken, informal Russian.

Because Раз ты уже дома is a subordinate clause that gives the reason for the request.

So the sentence is built like this:

• Раз ты уже дома, = since you’re already home,
• помоги мне донести коробку до кухни. = help me carry the box to the kitchen.

Russian normally separates this kind of clause with a comma.

Yes, Russian often omits subject pronouns when they are clear from context. But here ты is included for clarity or slight emphasis:

• Раз ты уже дома... = since you are already home...
• Раз уже дома... = since you’re already home... (more compressed, more colloquial)

Keeping ты makes the sentence sound a little more explicit and natural for a direct request.

Уже means already, and дома means at home.

So ты уже дома = you’re already home.

A useful point: in this phrase, дома functions like an adverbial expression meaning at home, not like a regular noun phrase you need to analyze deeply every time. It’s best to learn быть дома as a common chunk: to be at home.

Помоги is the imperative of the perfective verb помочь. It is used for one specific completed action:

• Помоги мне... = help me (with this one thing)

Помогай is the imperfective imperative and usually sounds more like:

• help out
• be helping
• go ahead and help
• help regularly / in an ongoing way

In this sentence, the speaker is asking for one concrete favor, so помоги is the natural choice.

Because помочь / помогать takes the person being helped in the dative:

• помоги мне = help me
• помоги ему = help him
• помоги нам = help us

So in помоги мне донести коробку, мне means help me carry the box.

Донести is a perfective verb with the prefix до-, which often gives the idea of carrying something all the way to a destination or finishing the carrying.

Compare:

• нести = to carry, to be carrying
• донести = to carry all the way to, to finish carrying to a place

So:

• помоги мне нести коробку = help me carry the box
• помоги мне донести коробку = help me carry the box the rest of the way / all the way to the destination

In this sentence, донести suggests reaching the kitchen as the endpoint.

Because коробка is the direct object of донести:

• донести что? = carry what?
• коробку = the box

So the nominative коробка changes to accusative коробку.

До requires the genitive case, so:

• кухня → кухни

That is why we get до кухни.

Literally, до кухни means up to the kitchen / as far as the kitchen. In English, this is often translated simply as to the kitchen.

A learner may wonder why this is not на кухню. In Russian:

• на кухню = into/to the kitchen
• до кухни = up to the kitchen, as far as the kitchen

In real usage, до кухни can sound natural when the focus is on getting the box to that destination point, especially from somewhere else in the home.

Not exactly, because the meaning and tone would change.

• раз = since / seeing that
  This assumes the fact is already true and uses it as a reason.
• если = if
  This introduces a condition, not a known fact.
• потому что / так как / поскольку = because / since
  These are possible in other sentence structures, but they do not sound exactly the same.

For example:

• Раз ты уже дома, помоги мне... = Since you’re already home, help me...
• Если ты уже дома, помоги мне... = If you’re already home, help me...
  This sounds conditional: maybe you are, maybe you aren’t.

So раз is the best choice here because the speaker treats you’re already home as established.

Putting Раз ты уже дома first sets up the reason before the request. This is very natural in both Russian and English:

• Since you’re already home, help me...

It can also be moved:

• Помоги мне донести коробку до кухни, раз ты уже дома.

That version is possible too, but it puts the request first and the reason afterward. The original order sounds very natural and conversational.