Learn Farsi: در سونا گرمت است چون بخار زیاد است. - You are warm in the sauna because there is a lot of steam.

In Farsi, physical feelings are expressed differently than in English. Instead of saying you are warm, Farsi phrases it more like the warmth belongs to you. The adjective گرم takes the second-person singular enclitic ـَت to become گرمت (your warm), followed by the 3rd person verb است.

Why is the verb است used here instead of the second-person form هستی?

With psychological or experiencer verbs, the sensation itself acts as the grammatical subject, not the person feeling it. Because of this, the verb is always in the 3rd person singular. You change who is feeling the sensation by swapping the attached pronoun, such as changing گرمت است to گرمم است to say I am warm.

Can I just say تو گرم هستی to mean you are warm?

Using تو گرم هستی would sound very strange here. It implies you are describing your physical temperature to the touch, like a warm object, or describing a warm personality. To describe the internal feeling of being hot or cold, you must use the experiencer structure.

How would this sentence sound in everyday spoken Farsi?

In casual speech, the verb است often attaches to the previous word and turns into an 'eh' sound. So گرمت است becomes گرمته and زیاد است becomes زیاده. The whole sentence would be spoken as در سونا گرمته چون بخار زیاده.