Learn Turkish: Tabiat burada dingin mi dingin gökyüzü ile buluşuyor.

This is the special emphatic structure introduced in this lesson. By repeating an adjective and placing the question particle (mı/mi/mu/mü, following 4-way vowel harmony) between them, you intensify the adjective's meaning. It stops being a question and instead acts like 'very', 'truly', or 'exceptionally'. Here, dingin (serene) becomes dingin mi dingin (very serene).

Why is ile used after gökyüzü instead of the accusative case (gökyüzünü)?

In English, you 'meet someone' or 'meet the sky' using a direct object. However, in Turkish, the verb buluşmak (to meet / to come together) requires the postposition ile (or its suffixed forms -la/-le). You are literally saying nature meets with the sky. This is a common pattern for Turkish reciprocal verbs that involve mutual interaction.

How do we know dingin mi dingin describes the sky and not nature?

In Turkish, adjectives and adjectival phrases are placed directly in front of the noun they modify. Since dingin mi dingin sits right in front of gökyüzü, it forms the single phrase 'the very serene sky'. If it were meant to describe nature, the sentence would start with Dingin mi dingin tabiat...

Could I just use a more common word like sakin instead of dingin here?