Learn Swedish: Grannarna respekterar varandras gränser. - The neighbors respect each other's boundaries.

It comes from the reciprocal pronoun varandra which means each other. Just like in English, we add an s to make it possessive, so varandras translates directly to each other's.

Why do we use the indefinite plural gränser instead of the definite gränserna here?

In Swedish, whenever you use a possessive word like min, deras, or the genitive varandras, the noun that follows must always be in its indefinite form. So it is varandras gränser, never varandras gränserna.

What is the singular form of grannarna, and why does the spelling change?

The singular is en granne (a neighbor). Because it ends in an -e, it drops that vowel when adding the plural ending -ar. This makes it grannar (neighbors) and finally grannarna (the neighbors) in the definite form.

Can gränser mean physical property lines, or does it mean personal boundaries?

It can mean both! The singular noun en gräns translates to a border, limit, or boundary. Depending on the context between these neighbors, they could be respecting physical property lines or respecting personal privacy.