There is inconsistency in this section (including the pictures) about when "y" is used and when you expect "the" to be used in English.
There is a difference in the use of the definite article in English and Welsh in these examples.
The expression 'In Spring, In Summer etc' would always need the definite article in Welsh eg Yn y Gwanwyn, Yn yr Haf....
Conversely, yn y gwanwyn could be translated into English as "in spring".
And so y gwanwyn could be translated into English as "spring".
Because this exercise asks to translate y gwanwyn and I answered, "spring", which was rejected -- it told me to type "the spring".
I think "spring" without article should also be accepted here because, as you say, there is a difference in the use of the definite article in English and Welsh.