Learn Turkish: Yeni telefon aldım. Güle güle kullan! - "I bought a new phone." "Enjoy your new one!"

In Turkish, when the focus is on the type of thing you bought rather than the quantity, the word bir is often dropped. Saying yeni telefon aldım is very natural and focuses on the action of getting a new phone, rather than getting exactly one phone.

I learned that güle güle means goodbye. What does it mean here?

Its literal meaning is 'smilingly' or 'happily.' It is formed by repeating the verb gülmek (to laugh or smile) with the adverb suffix -e. When someone leaves, you tell them to go 'smilingly.' Here, you are telling them to use their new item 'smilingly' (happily).

When exactly should I use the phrase güle güle kullan?

It is a set phrase used whenever someone gets a new item, like a phone, a car, or furniture. If they bought new clothes or shoes, you would instead say güle güle giy (wear it happily).

How do I make this phrase formal or plural?

Because kullan is a command for a singular, informal 'you', you just change the verb ending. For formal situations or when speaking to multiple people, you would say güle güle kullanın.