"The forest has fifteen trees."

Translation: Το δάσος έχει δεκαπέντε δέντρα.

[omikros]

Μα είναι μικρό αυτό το λεγόμενο <δάσος>!

[jaye16]

Yeah, so it would seem :-0

[bonbayel]

15 trees does not make a forest, but a wood/woods perhaps.

[Marva441926]

Fifteen trees make a grove. Not a forest.

[jaye16]

Good catch I'll mark it for use in the new tree. Thank you.

[Marva441926]

Does Greek have a word for a grove of trees?

[Dimitra956826]

There is the word άλσος that's used for a grove, at least most of the times. ^.^ Περιβόλι is for an orchard.

[Vemund63]

The word δεντρα is pronounciated something like "dedra". Could I also write πετρα as πεντρα?

[mizinamo]

The word δεντρα is pronounciated something like "dedra".

The first sound should be a /ð/ sound as in "then", not a /d/ sound as in "den" -- ðedra not dedra.

Could I also write πετρα as πεντρα?

No. πέτρα has a /t/ sound in the middle.