Если домофон снова зазвонит, открой, пожалуйста, дверь.

Зазвонит is the perfective future of зазвонить, which means to start ringing.

In this sentence, the idea is not if the intercom is ringing in general, but if it rings / starts ringing again at some point in the future.

• звонит = is ringing / rings
• будет звонить = will be ringing / will ring repeatedly or as a process
• зазвонит = will start ringing

So Если домофон снова зазвонит... means something like If the intercom rings again...

Russian commonly uses the future tense after если when talking about a real future condition.

So Russian says:

• Если домофон зазвонит... = If the intercom rings...

Even though English normally avoids will in this kind of clause, Russian does not. This is completely normal Russian grammar.

A useful pattern is:

• Если + future, imperative / future / present

For example:

• Если он придёт, скажи мне. = If he comes, tell me.
• Если будет дождь, останемся дома. = If it rains, we’ll stay home.

The prefix за- often gives the meaning to begin doing something.

So:

• звонить = to ring / to call
• зазвонить = to start ringing

That is why зазвонит fits well here: the intercom is expected to begin ringing again.

This use of за- is common with sounds and actions:

• запеть = to start singing
• закричать = to start shouting
• засмеяться = to start laughing

Открой is the imperative of the perfective verb открыть, and it usually means do it once / complete the action.

Here, the speaker wants one completed action:

• open the door

So открой is the natural choice.

Compare:

• открой дверь = open the door (once, now, as a complete action)
• открывай дверь = open the door / start opening the door / be opening the door

The imperfective imperative открывай can sound more like:

• begin doing it now,
• do it as an ongoing action,
• or sometimes a repeated/habitual instruction in the right context.

For this sentence, открой is the standard option.

Дверь is in the accusative case, because it is the direct object of открыть: you open what? — дверь.

For feminine nouns ending in -ь, the accusative singular is often the same as the nominative singular, so the form does not change:

• nominative: дверь
• accusative: дверь

So even though the form looks the same, its role in the sentence is different.

In Russian, домофон can naturally mean the intercom device itself, and that device can ring.

So:

• домофон зазвонит = the intercom rings

This is similar to English phrases like:

• The phone is ringing
• The doorbell rang

The sentence is focusing on the sound of the device, not on the person outside.

Снова means again.

Here it modifies зазвонит:

• Если домофон снова зазвонит... = If the intercom rings again...

Its placement is flexible, but different positions can slightly change emphasis:

• Если домофон снова зазвонит... = neutral, very natural
• Если снова зазвонит домофон... = more emphasis on the event happening again
• Если домофон зазвонит снова... = also possible, but a bit less neutral here

The version in the sentence is probably the most natural everyday order.

Пожалуйста means please.

Placed inside the command:

• открой, пожалуйста, дверь

it softens the imperative and makes it polite.

Russian often places пожалуйста in the middle of the sentence like this, especially with requests:

• Скажи, пожалуйста, ...
• Передай, пожалуйста, ...
• Открой, пожалуйста, ...

You could also say:

• Пожалуйста, открой дверь.

But the version in your sentence sounds especially natural in everyday speech.

Открой is the singular informal imperative, used when speaking to:

• one person,
• someone you address with ты.

Откройте is used for:

• one person formally (вы),
• or more than one person.

So the sentence suggests the speaker is talking casually to one person.

Compare:

• Открой дверь. = open the door. (informal singular)
• Откройте дверь. = open the door. (formal singular or plural)

Not exactly.

• Если домофон снова зазвонит... = If the intercom rings again...
• Когда домофон снова зазвонит... = When the intercom rings again...

Если presents it as a condition or possibility. Когда presents it as something expected to happen.

So the difference is:

• если = maybe it will happen
• когда = it will happen, and when it does...

Both are grammatical, but they express different attitudes.

The word order is fairly flexible, but the original version is natural and neutral:

• Если домофон снова зазвонит, открой, пожалуйста, дверь.

Some possible variations:

• Если снова зазвонит домофон, открой, пожалуйста, дверь.
• Открой, пожалуйста, дверь, если домофон снова зазвонит.

These are still understandable, but they shift emphasis a little.

Russian word order often depends on what the speaker wants to highlight, not just on strict syntax. For a learner, the original sentence is a very good standard model.

Домофон is the intercom system/device. Звонок can mean doorbell, ring, or phone call, depending on context.

So:

• домофон зазвонил = the intercom rang
• звонок в дверь = the doorbell / a ring at the door
• телефонный звонок = a phone call

In this sentence, домофон is specific: the speaker means the building or apartment intercom, not just any bell or ring.