Коли я чую дверний дзвінок, я одразу відкриваю двері.

Here коли means when. In this sentence it introduces a time clause: When I hear the doorbell, I immediately open the door.

Because the sentence describes a repeated action, whenever is also a good English sense. It is not if here. For if, Ukrainian would normally use якщо.

They are present tense because the sentence describes a habitual or repeated action: this is what I do every time the situation happens.

Both verbs are also imperfective, which fits an ongoing or repeated pattern.

If you wanted a single future situation, Ukrainian would normally use perfective future forms: Коли я почую дверний дзвінок, я одразу відкрию двері.

Чую is the 1st person singular present form of чути: to hear.

So:

• я чую = I hear
• ти чуєш = you hear
• він / вона чує = he / she hears

Because there are two clauses, and Ukrainian can state the subject in each one:

• Коли я чую дверний дзвінок
• я одразу відкриваю двері

This is completely natural. Ukrainian can also omit subject pronouns when the meaning is clear, so a shorter version is possible: Коли чую дверний дзвінок, одразу відкриваю двері.

The repeated я just makes the sentence a bit clearer or more explicit.

Because Коли я чую дверний дзвінок is a subordinate clause, and Ukrainian normally separates it from the main clause with a comma.

So the structure is:

• subordinate clause: Коли я чую дверний дзвінок
• main clause: я одразу відкриваю двері

Дверний дзвінок means doorbell.

The parts are:

• дверний = door-, related to a door
• дзвінок = bell, ring, call

So together it literally means something like door bell.

Дверний is an adjective, and it must agree with дзвінок.

Here дзвінок is:

• masculine
• singular
• inanimate

So the adjective also appears in the matching masculine singular form: дверний.

Because дзвінок is the direct object of чую, so it is in the accusative case.

But for many inanimate masculine nouns in Ukrainian, the accusative form is the same as the nominative form. So:

• nominative: дзвінок
• accusative: дзвінок

That is why it looks unchanged.

A useful comparison:

• Я бачу стіл = I see a table
• Я бачу брата = I see my brother

With an animate masculine noun like брат, the accusative changes.

Because двері is a plural-only noun in Ukrainian. Even when English uses singular door, Ukrainian normally uses двері.

So Ukrainian treats it grammatically as plural:

• ці двері = this door / these doors, depending on context
• двері відчинені = the door is open

This is something learners just need to get used to.

It is in the accusative case, because it is the direct object of відкриваю.

However, for inanimate plural nouns, the accusative is usually the same as the nominative. So двері looks the same here.

Yes. Both are natural.

• відкривати / відкрити = to open in a general sense
• відчиняти / відчинити = to open, especially for things like doors and windows

So відчиняю двері is often especially idiomatic for opening a door, but відкриваю двері is also correct and common.

Yes. Ukrainian word order is fairly flexible.

The original sentence is neutral and clear: Коли я чую дверний дзвінок, я одразу відкриваю двері.

But you could also say:

• Я одразу відкриваю двері, коли чую дверний дзвінок.
• Коли я чую дверний дзвінок, одразу відкриваю двері.

The basic meaning stays the same, but the emphasis can shift slightly.

Because Ukrainian has no articles. There is no direct equivalent of English a or the.

Whether something is definite or indefinite is usually understood from context. So:

• дверний дзвінок can mean a doorbell or the doorbell
• двері can mean a door or the door

Here the context makes the meaning clear.