Когда пришёл код подтверждения, я вошла в приложение и оплатила заказ.

In this sentence, когда can be understood as when, but in natural English it often feels like once or after because it introduces the event that triggers the next actions.

So:

• Когда пришёл код подтверждения... = When / Once the confirmation code arrived...

Russian often uses когда in this kind of sequence without needing a special word for once.

Yes — this is very natural Russian.

Russian commonly uses прийти / приходить for things like:

• сообщение пришло = the message arrived
• письмо пришло = the email/letter arrived
• код пришёл = the code arrived

So пришёл here does not mean the code literally walked somewhere. It means arrived / came through.

Because the subject is код, and код is a masculine noun.

In the past tense, Russian verbs agree with gender and number:

• masculine: пришёл
• feminine: пришла
• neuter: пришло
• plural: пришли

So:

• код пришёл
• посылка пришла
• письмо пришло
• данные пришли

Because the speaker is female.

In the Russian past tense, verbs show gender in the singular:

• я вошёл, я оплатил = a man is speaking
• я вошла, я оплатила = a woman is speaking

So я вошла в приложение и оплатила заказ tells you the speaker is female.

This is very important for English speakers, because English past tense does not show this kind of agreement.

Because the sentence describes completed, one-time actions in sequence.

• пришёл = the code arrived
• вошла = she logged in / entered
• оплатила = she paid

Perfective verbs are the normal choice when you are talking about:

• one completed event
• a clear result
• a sequence of finished actions

If you used imperfective forms, the meaning would change and sound more like process, repetition, or background action.

Войти в приложение means to enter / log into the app.

It focuses on the action of getting in.

By contrast:

• была в приложении
• находилась в приложении

mean was in the app and describe a state, not the act of entering.

So here вошла is the right choice because first the code arrived, then she logged in, then she paid.

Because войти takes в + accusative when it means movement into something.

• войти в приложение = to enter the app
• войти в комнату = to enter the room

But if you mean location, you use в + prepositional:

• в приложении = in the app
• в комнате = in the room

So:

• Я вошла в приложение = I entered/logged into the app
• Я была в приложении = I was in the app

This is a very common Russian noun + noun pattern.

код подтверждения literally means code of confirmation, which in English becomes confirmation code.

The second noun often goes into the genitive to define the first noun:

• номер телефона = phone number
• страница книги = page of the book / book page
• центр города = city center

So подтверждения is genitive singular after код.

Because оплатить and заплатить are used differently.

• оплатить что?

  • accusative
  • оплатить заказ
  • оплатить счёт
  • оплатить билет

• заплатить за что?

  • accusative with за
  • заплатить за заказ
  • заплатить за билет

So in this sentence, оплатила заказ is exactly right.

A very common learner mistake is заплатить заказ, which is not standard Russian.

It is in the accusative singular, because it is the direct object of оплатила.

But заказ is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: заказ
• accusative: заказ

That is why there is no visible ending change.

Because that whole part is a subordinate clause introduced by когда.

Russian normally separates subordinate clauses with a comma:

• Когда пришёл код подтверждения, я вошла в приложение и оплатила заказ.

This is the standard pattern:

• Когда X, Y.
• Если X, Y.
• Потому что X, Y.

So the comma is required here.

Yes, sometimes it could be omitted if the subject is clear from context:

• Когда пришёл код подтверждения, вошла в приложение и оплатила заказ.

But in isolation, keeping я is more natural and clearer.

Also, in the past tense, Russian verbs do not show person, only gender and number. So вошла tells us the subject is one female person, but not automatically I rather than she unless the context makes it obvious.

That is one reason why я is useful here.