Buzdolabındaki yoğurt çok soğuk.

Because Turkish often packs a whole phrase into one word with suffixes.

buzdolabındaki can be broken down like this:

• buzdolabı = refrigerator / fridge
• -nda = in / inside
• -ki = the one that is in / the one located in

So buzdolabındaki yoğurt means something like:

• the yogurt in the fridge
• literally, the yogurt that is in the fridge

This is very normal in Turkish.

This is because of consonant softening.

The noun dolap ends in p, and when a vowel-initial suffix is added, that p often changes to b:

• dolap → dolabı
• kitap → kitabı
• ağaç → ağacı

So in buzdolabı, the p becomes b before the suffix.

-ki turns a location-based expression into something that acts like an adjective.

• buzdolabında = in the fridge
• buzdolabındaki = the one in the fridge / the one that is in the fridge

So it lets buzdolabındaki modify yoğurt.

Similar examples:

• masadaki kitap = the book on the table
• çantadaki telefon = the phone in the bag

It is a very useful suffix in Turkish.

That n is a linking consonant that appears before case endings in this kind of noun.

Here, buzdolabı behaves like a compound form, and when the locative ending -da / -de is added, it becomes:

• buzdolabı
  • -nda → buzdolabında

So you get buzdolabında, not buzdolabıda.

This n is common after forms that already contain a possessive-type ending.

In Turkish, the present-tense to be is often not stated in simple 3rd-person sentences.

So:

• Yoğurt çok soğuk. = The yogurt is very cold.

There is no separate word for is here, but the meaning is understood automatically.

You may also see soğuktur, which is more formal or general in tone, but soğuk by itself is the normal everyday form here.

Because çok is modifying the adjective soğuk.

• çok = very
• soğuk = cold

So:

• çok soğuk = very cold

In Turkish, words like çok usually come before the adjective or adverb they modify.

Also note:

• with adjectives/adverbs, çok often means very
• with nouns, çok often means many / much / a lot of

Turkish does not have a direct equivalent of the English definite article the.

Definiteness is usually understood from:

• context
• word order
• suffixes
• whether the noun is specific

In this sentence, buzdolabındaki yoğurt already points to a specific yogurt, so English naturally uses the even though Turkish does not say it separately.

The normal structure is:

• [buzdolabındaki yoğurt] [çok soğuk]

That is:

• subject/topic first
• predicate at the end

Also, in Turkish, modifiers come before the noun:

• buzdolabındaki
  • yoğurt
• literally: the in-the-fridge yogurt

So the sentence follows very typical Turkish word order.

The letter ğ in modern Turkish usually does not sound like a hard English g.

In yoğurt, it usually:

• lengthens the previous vowel a bit, or
• creates a very light glide

So yoğurt is pronounced roughly like:

• yo-urt
• or yoo-urt

not like yog-gurt.

The same idea appears in soğuk, which sounds roughly like:

• so-uk
• or soo-uk

The exact sound can vary a little depending on accent and speaking speed.

They are not the same structure.

• buzdolabındaki yoğurt = the yogurt in the fridge

  • here, buzdolabındaki directly modifies yoğurt

• buzdolabında yoğurt = more like yogurt in the fridge

  • this is just a location phrase plus a noun
  • by itself, it does not work the same way as the adjective-like -ki form

So if you want to say the yogurt that is in the fridge, you need:

• buzdolabındaki yoğurt

Yes, yoğurt is singular in form here.

But also remember that yoğurt is often treated as a mass noun, like water or milk in English. So even when talking about a quantity of yogurt, Turkish often keeps it singular.

If you really mean separate yogurts, you could use the plural:

• yoğurtlar = yogurts

For example:

• Buzdolabındaki yoğurtlar çok soğuk. = The yogurts in the fridge are very cold.

Notice that the adjective soğuk does not change for singular or plural.