Learn Hardcore Turkish: Tedavi pahalı.

Turkish does not have definite or indefinite articles. If you want to emphasize “a treatment,” you can add bir: bir tedavi. Otherwise tedavi alone can mean “treatment” or “the treatment” based on context.

Why isn’t there any case ending on tedavi to mark it as the subject?

In simple Turkish sentences with a predicate adjective, the subject stays in the nominative case without a suffix. You only add case endings when you need to mark direct objects, possession, etc.

Why is there no verb “to be” in Tedavi pahalı?

In Turkish, the present-tense copula (–dır/–dir/–dur/–dür) is often omitted in speech and informal writing. An adjective following a noun acts as the predicate. So Tedavi pahalı literally means “Treatment is expensive.”

When would I use pahalıdır instead of pahalı?

Pahalıdır is the formal/written 3rd-person singular copula form of the adjective. You’ll see it in formal texts, news, or official documents:
“Tedavi pahalıdır.”
In everyday conversation, speakers usually drop the copula and say Tedavi pahalı.

How do I ask “Is the treatment expensive?” in Turkish?

Add the question particle mı (with vowel harmony) after pahalı:
Tedavi pahalı mı?

How can I say “The treatment is not expensive”?

Use değil after the adjective to negate:
Tedavi pahalı değil.

How do I express “very expensive” or “quite expensive”?

Use an adverb like çok (“very”) or oldukça (“quite”):
Tedavi çok pahalı.
Tedavi oldukça pahalı.

What is the origin and structure of the word pahalı?

Pahalı is formed from the noun paha (“price, value”) plus the adjective-forming suffix -lı (“having”). So it literally means “having price,” i.e. “expensive.”