Yeni telefonunu güle güle kullan.

It is a very common Turkish well-wishing phrase. You say it to someone who has bought or received something new. Literally, it is something like use it happily / smilingly, but in natural English it works more like Enjoy it or Use it in good health.

So even though kullan is grammatically an imperative, the whole sentence is really a friendly wish, not a harsh command.

telefonunu can be broken down like this:

• telefon = phone
• -un = your
• -u = accusative ending

So here it means your phone as the object of the verb.

One useful extra note: this exact form can also mean his/her phone in some other contexts, but in this sentence the meaning is clearly your phone.

Because kullanmak means to use, and here phone is the direct object of the verb.

In Turkish, a specific direct object usually takes the accusative ending. So:

• telefonun = your phone
• telefonunu = your phone as the specific thing being used

That is why the extra -u appears.

kullan is the 2nd person singular informal imperative of kullanmak (to use).

So it is the form used when speaking directly to one person in an informal way:

• kullan = use

Turkish usually leaves out the subject pronoun, so sen is not needed. The sentence already clearly means you.

You would say:

Yeni telefonunuzu güle güle kullanın.

Here:

• telefonunuzu = your phone with polite/plural you
• kullanın = polite/plural imperative

So this version can be used for one person politely or for more than one person.

Because Turkish adjectives normally come before the noun, just like in English.

So:

• yeni telefon = new phone

Then the possessive and case endings go on the noun, not on the adjective:

• yeni telefonunu = your new phone

Because Turkish often places the verb at the end of the sentence.

So in:

Yeni telefonunu güle güle kullan

the order is:

• yeni telefonunu = object
• güle güle = adverbial expression
• kullan = verb

This is the most normal and natural word order here.

Yes, it is the same pair of words.

By itself, Güle güle! is a goodbye said by the person who stays behind to the person who is leaving.

But in güle güle kullan, it becomes part of a fixed expression meaning something like enjoy using it. So the context changes how it is understood.

It is a fixed idiomatic pattern. Turkish often uses repetition to make adverbial expressions that sound warm, vivid, or expressive.

Here güle güle gives the sense of doing something cheerfully and with enjoyment. It is best to learn it as one chunk rather than trying to translate each word separately every time.

Yes. This is very common for new possessions. For example:

• Arabanı güle güle kullan. = Enjoy your new car.
• Bilgisayarını güle güle kullan. = Enjoy your new computer.

So the same pattern works well when someone gets something new that they will use. For other kinds of things, Turkish may prefer a different verb, but the idea of giving a warm good wish is the same.