Questions & Answers about Johan har Tomas hatt.
Why doesn't Tomas get an extra 's' or an apostrophe to show possession?
In Swedish, you never use an apostrophe to show ownership. Furthermore, if a name already ends in an 's' (like Tomas, Lars, or Anders), you do not add another 's'. The name stays exactly the same, doing double duty as both the name and the possessive form.
Why is it hatt and not hatten (the hat) since we are talking about a specific hat?
In Swedish, whenever a noun follows an owner, it must drop its definite ending and use the indefinite (basic) form. So even though it is a specific hat, it is always Tomas hatt and never Tomas hatten.
Since Tomas doesn't change, how do I know it means "Tomas's" and not just the name?
You can tell entirely by the word order. Whenever a name is placed directly in front of a noun (like hatt), it acts as the owner. The listener knows that the first person owns the following object.
Can I say that Johan has "the hat of Tomas" to avoid this rule?
No, Swedish does not use a preposition like "of" for personal ownership. You must use this owner-first structure where the owner comes right before the thing they own.