Лікар торкнувся вилиці.

Because the verb here is торкнутися, which normally includes -ся as part of the verb.

• торкнутися = to touch
• past masculine singular: торкнувся

So -ся is not optional here. In this verb, it is simply part of the standard dictionary form. A learner should memorize торкнутися as a whole verb.

Because торкнутися takes the genitive case, not the accusative.

• nominative: вилиця
• genitive singular: вилиці

So:

• Лікар торкнувся вилиці. = The doctor touched the cheekbone.

This is a very common pattern in Ukrainian:

• торкнутися чого? = to touch what? → genitive

That is why вилицю would sound wrong here.

Here it is singular: the genitive singular of вилиця.

The form вилиці can look confusing, because the same spelling can match other case/number patterns in other contexts. But in this sentence, after торкнувся, it is understood as genitive singular.

So the sentence means that the doctor touched one cheekbone / the cheekbone area, not multiple cheekbones.

Not exactly. Вилиця means cheekbone or the bony part of the face around that area.

If you want cheek in the softer, everyday sense, that is usually щока.

So:

• вилиця = cheekbone
• щока = cheek

That distinction matters, because in English touch the cheek and touch the cheekbone are not quite the same.

The past-tense verb shows that the subject is grammatically masculine:

• торкнувся = masculine past
• торкнулася = feminine past

So Лікар торкнувся вилиці. means the doctor is grammatically male.

If the doctor were female, you would say:

• Лікар торкнулася вилиці.

Notice that лікар itself can still be used, but the verb changes to show gender in the past tense.

Yes, there can be.

Ukrainian often leaves possession unstated when it is obvious from context. So this sentence could mean something like:

• the doctor touched the cheekbone
• the doctor touched his cheekbone
• the doctor touched her cheekbone
• the doctor touched the patient’s cheekbone

The exact interpretation depends on context. Ukrainian often does not spell that out unless needed.

If you wanted to be explicit, you could add a possessive word, for example:

• його вилиці = his cheekbone
• її вилиці = her cheekbone

The infinitive is торкнутися.

This is a perfective verb, so it presents the action as a single completed event:

• торкнутися = to touch / to make contact once
• торкався would be the imperfective past form, from торкатися

Compare:

• Лікар торкнувся вилиці. = the doctor touched the cheekbone (completed action)
• Лікар торкався вилиці. = the doctor was touching / used to touch the cheekbone

So in your sentence, the action is seen as completed.

Because Ukrainian has no articles like a or the.

So лікар can mean:

• a doctor
• the doctor

And вилиці here can mean:

• a cheekbone
• the cheekbone

Context tells you which one is meant. In this sentence, English would usually translate it with the doctor and the cheekbone, but Ukrainian does not need separate article words.

Yes. Ukrainian word order is fairly flexible because case endings do a lot of the grammatical work.

The neutral order here is:

• Лікар торкнувся вилиці.

But other orders are possible for emphasis, for example:

• Вилиці торкнувся лікар.
• Торкнувся вилиці лікар.

These alternatives are more marked and depend on context, style, or what the speaker wants to emphasize. For a learner, the original order is the most straightforward and natural.

Yes, it is natural.

It sounds like something you might hear in a medical, descriptive, or narrative context. For example, a doctor examining a patient might touch the cheekbone area.

The sentence is short, grammatical, and idiomatic:

• Лікар = subject
• торкнувся = past-tense verb
• вилиці = genitive object required by the verb

So it is a good example of how торкнутися + genitive works in real Ukrainian.