Если завтра придут гости, мы повесим гирлянду у окна и быстро наведём порядок.

Russian and English work differently here.

In Russian, when the condition is about the future, it is normal to use a future verb after если:

• Если завтра придут гости, мы...

In English, we usually use the present after if:

• If guests come tomorrow, we’ll...

So придут is not strange in Russian at all. It is the standard way to talk about a real future condition.

Because придут is the correct future form here, and it also matches the meaning of a single completed arrival.

• придут = will come / will arrive
• приходят = come / are coming in the present, or come regularly

With завтра, Russian normally wants a future form, not a present habitual form.

Also, прийти → придут is perfective, which fits a one-time event: the guests arrive, and then you react.

Yes, you could, but the meaning shifts a little.

• Если завтра придут гости focuses on the event of their coming/arriving.
• Если завтра будут гости focuses more on the fact that there will be guests or that you will have company.

So both can work in many situations, but придут гости is more specifically about arrival.

Because they describe what we will do if the condition is met.

• повесим = we will hang / put up
• наведём = we will put in order / tidy up

These are both perfective future forms. Russian often uses perfective future when the action is seen as a completed result:

• повесим гирлянду = we will get the garland hung up
• наведём порядок = we will get things tidied up

If you wanted to focus on process instead, you might use forms like:

• будем вешать
• будем наводить порядок

But that is not the idea here.

Because it is the direct object of повесим, so it takes the accusative case.

Dictionary form:

• гирлянда

Accusative singular:

• гирлянду

So:

• Мы повесим гирлянду = We will hang the garland / a garland

This is a very common pattern in Russian: a direct object changes case.

Окна here is genitive singular because it comes after у.

The preposition у often means by, near, or at.

• у окна = by the window / near the window

Dictionary form:

• окно

After у:

• у окна

So the case change is caused by the preposition.

Because навести порядок is a very common Russian expression meaning:

• to tidy up
• to put things in order

Literally, it is something like bring about order, but in normal English you would translate it as tidy up.

So:

• быстро наведём порядок = we’ll quickly tidy up

This is a set phrase worth learning as a whole.

It is in the accusative, but for many inanimate masculine nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: порядок
• accusative: порядок

Even though the form does not change, the case still does.

Because Если завтра придут гости is a subordinate clause introduced by если, and Russian separates it from the main clause with a comma.

So the structure is:

• Если завтра придут гости, = if guests come tomorrow
• мы повесим... = we will hang...

This comma is required in standard Russian spelling.

Russian word order is flexible, but different orders can sound slightly different.

• Если завтра придут гости sounds natural and neutral here.
• Если завтра гости придут is possible, but it gives more emphasis or contrast to гости.

Russian often puts the verb before the subject when presenting an event:

• придут гости = guests will come / guests will arrive

So this order feels very natural in this sentence.

It modifies the whole action наведём порядок:

• быстро наведём порядок = quickly tidy up

You can move adverbs around in Russian more freely than in English, but this position is very normal and neutral.

For example, мы наведём порядок быстро is possible, but it sounds less neutral in this context.

Yes. Russian often omits subject pronouns when they are clear from the verb ending.

• повесим already tells you it means we will hang
• наведём already tells you it means we will tidy up

So мы is not strictly necessary. It is included here for clarity or emphasis.

Because ё is a separate letter in Russian, and it shows both pronunciation and stress.

• наведём is stressed on -дём

In many Russian texts, ё is sometimes written as е, especially in casual or less careful printing, but learner materials often keep ё because it is helpful.

So seeing наведём with ё is actually very useful.