Parktaki köpek çok sakin.

Parktaki means the one in the park or the one that is in the park.

It is built like this:

• park = park
• parkta = in the park
• parkta + ki → parktaki

The suffix -ki can turn a location phrase into something that describes a noun. So:

• parktaki köpek = the dog in the park

You can think of -ki here as roughly similar to that is in.

This is because of consonant harmony in Turkish.

The locative ending has two main forms:

• -da / -de
• -ta / -te

After a voiceless consonant like k, p, t, ç, f, h, s, ş, Turkish usually uses -ta / -te.

Since park ends in k, you get:

• park + ta = parkta = in the park

Then:

• parkta + ki = parktaki

In Turkish, words that describe a noun usually come before the noun.

So:

• parktaki köpek = the dog in the park
• literally, something like the in-the-park dog

This is normal Turkish word order for noun phrases.

Other examples:

• büyük ev = big house
• okuldaki öğretmen = the teacher at the school / the teacher in the school

Turkish does not have definite and indefinite articles like English the and a/an.

Whether something means a dog or the dog depends on context.

So köpek can mean:

• dog
• a dog
• the dog

In Parktaki köpek çok sakin, the phrase parktaki makes the dog more specific, so in English it is usually translated as the dog in the park.

In Turkish, the verb to be is often not expressed in simple present-tense sentences like this one.

So:

• Köpek sakin. = The dog is calm.
• Köpek çok sakin. = The dog is very calm.

Turkish often leaves out a separate word for is in this kind of sentence.

In more formal or emphatic styles, you may sometimes see forms like sakindir, but in everyday speech sakin by itself is completely normal.

Here, çok means very.

So:

• çok sakin = very calm

Yes, çok can also mean many / much, depending on what it is modifying.

Examples:

• çok güzel = very beautiful
• çok köpek = many dogs

So the meaning depends on the structure:

• before an adjective/adverb → often very
• before a noun → often many/much

In this sentence, since sakin is an adjective, çok means very.

In this sentence, sakin is an adjective meaning calm or quiet.

So:

• köpek sakin = the dog is calm

But in other contexts, sakin can sometimes have related meanings such as peaceful, quiet, or gentle, depending on what is being described.

For example:

• sakin bir yer = a quiet / peaceful place
• sakin biri = a calm person

So the exact English word may vary a little by context.

Because köpek is the subject of the sentence, it stays in its basic dictionary form.

• köpek = dog

Turkish adds case endings when a noun has a different role, such as direct object, location, possession, and so on.

For example:

• Köpek çok sakin. = The dog is very calm.
  
  • köpek is the subject

But:

• Köpeği gördüm. = I saw the dog.
  
  • köpeği is the definite direct object

So in your sentence, no extra ending is needed on köpek.

The sentence is:

• Parktaki köpek çok sakin.

Its structure is:

• Parktaki köpek = subject noun phrase
• çok sakin = predicate description

A natural breakdown is:

• [Parktaki köpek] [çok sakin].
• [The dog in the park] [is very calm].

Turkish often places the main descriptive part at the end, so this word order is very natural.

You could say Köpek parkta çok sakin, but it means something slightly different in emphasis and structure.

• Parktaki köpek çok sakin = The dog in the park is very calm.
  This identifies which dog you mean.

• Köpek parkta çok sakin = The dog is very calm in the park.
  This sounds more like you are saying where the dog is calm, not using in the park to identify the dog.

So if you want the dog in the park, parktaki köpek is the better form.

Yes. Turkish often allows this when the noun is understood from context.

For example:

• Parktaki çok sakin.

This can mean something like:

• The one in the park is very calm.

But in real use, this only works if it is already clear what kind of thing or person you are talking about. Otherwise, it can sound incomplete or vague.

So parktaki köpek is clearer for learners and in many everyday situations.