Why do I have to say ”a creionului”? The first suggestion from the hover translation was simply ”creionului” and I thought that was enough...
I cannot explain why, but it seems that if you would choose to say guma creionului then you should not be using an a. However, saying guma de șters a creionului requires that you use an a.
Maybe it's because guma de șters is a compound?
It could be. If I think of similar scenarios like
it follows your rule.
i have read that after an adjective you have to use Al. Ai, A, Ale... there are some rules to follow, oyu can download Romanian An Essencial Grammar, there is more explained.
If a modified noun has an indefinite article or an accusative adjective, the noun gets the possesive/genitive article.