Learn Icelandic: Hvar er sláttuvélin hans Jóns?

In spoken Icelandic, when talking about something belonging to a specific person, we use a structure often called the proprial article. Instead of just using the name, you add the possessive pronoun that matches the owner's gender (hans for a man) followed by the owner's name in the genitive case (Jóns).

Why does the object sláttuvélin have the definite article attached?

This is a strict rule when using this colloquial possessive structure. The owned object must always be in the definite form. Therefore, sláttuvél (lawnmower) takes the feminine definite suffix to become sláttuvélin before we add hans Jóns.

Would it be wrong to simply say Hvar er sláttuvél Jóns?

It is grammatically correct but sounds quite formal, like something you would read in a book or the news. In everyday conversation, native speakers almost always use the sláttuvélin hans Jóns pattern instead.

How do we know the gender of the lawnmower from this sentence?

We can see it is a feminine noun because it ends with the feminine definite article -in (sláttuvélin). Remember that the possessive pronoun hans is masculine because it matches the owner (Jón), not the feminine object being owned!