Після того як пацієнт зателефонує до реєстратури, він запишеться.

Після того як means after when it introduces a whole clause.

• після by itself is usually followed by a noun:
  • після дзвінка = after the call
• But if you want after
  • a full action with a verb, Ukrainian commonly uses:
    • після того як ...

So in this sentence, після того як пацієнт зателефонує... = after the patient calls...

Because зателефонувати is a perfective verb. In Ukrainian, the non-past form of a perfective verb usually has future meaning.

So:

• зателефонувати = perfective, to call once / to complete the call
• зателефонує = will call

By contrast:

• телефонувати = imperfective, to be calling / to call in general
• телефонує = is calling / calls

This is a normal difference between English and Ukrainian.

In Ukrainian, when both actions are in the future, it is very natural to use future forms in both clauses, especially with perfective verbs:

• Після того як пацієнт зателефонує, він запишеться.

So even though English prefers after he calls, Ukrainian uses after he will-call in form, though the meaning is completely natural.

Here запишеться means something like:

• will sign up
• will register
• in this medical context, most likely will make an appointment

The exact English wording depends on context, but the idea is that the patient gets their name entered for a visit.

The ending -ся marks a reflexive verb.

Here the pair is:

• записати = to write down / register someone
• записатися = to sign up / register oneself / make an appointment

So пацієнт запишеться means the patient is the one getting themself registered.

This does not always feel literally reflexive in English; often it is just part of the verb’s meaning.

Реєстратури is genitive singular.

That happens because the preposition до takes the genitive:

• реєстратура → до реєстратури

In this sentence, зателефонувати до реєстратури means to call the reception / registration desk.

In a clinic or hospital, реєстратура is the reception or patient registration desk.

It could sometimes be omitted, because Ukrainian often drops subject pronouns when the subject is clear.

But here він helps make the sentence easier to follow:

• Після того як пацієнт зателефонує до реєстратури, він запишеться.

The pronoun clearly shows that the person who will register is the same person: the patient.

Without він, the sentence is still understandable, but the explicit pronoun adds clarity.

Because the first part is a subordinate clause:

• Після того як пацієнт зателефонує до реєстратури = subordinate clause
• він запишеться = main clause

Ukrainian separates these clauses with a comma.

Also, notice that there is no comma inside після того як here. It functions as one conjunction unit in this sentence.

Yes, you could say коли, but the nuance changes a little.

• коли = when
• після того як = after

So:

• Коли пацієнт зателефонує до реєстратури, він запишеться
  = When the patient calls..., he will make an appointment
• Після того як... emphasizes that one action happens after the other

In this sentence, після того як makes the sequence especially clear.

Grammatically, пацієнт is a masculine noun.

It can mean:

• a male patient specifically, or
• a generic patient if the gender is not being emphasized

If you want to refer specifically to a female patient, you would usually say:

• пацієнтка

Then the pronoun would also change:

• Після того як пацієнтка зателефонує до реєстратури, вона запишеться.