This sentence is incorrect and has now been deleted.

The correct translation would be "The leaves of Spring" = "Dail Gwanwyn"

The way possession is translated from English to Welsh in this sort of sentence is to delete the first 'the' and the middle 'of' eg:-

The leaves of Spring = (after deletion) = leaves Spring = dail Gwanwyn

Here's an explanation:- https://www.youtube.com/watch?v=OUuvCwOHs2c

"Dail y Gwanwyn" is the title of a volume of poetry compiled by a school some time ago. Book titles etc often do not translate very well according to normal rules.

The Welsh pattern aaa y bbb where aaa and bbb are both nouns, as in the sentence given, usually translates literally as 'the aaa of the bbb' and often then needs to be rendered into better English:

• car yr athro - (the car of the teacher) - the teacher's car

This is explained in the course notes.