Learn French: Tu ne gardes qu'une possession. - You only keep one possession.

The structure ne ... que frames the verb to mean "only" (literally "nothing except"). The ne goes before the conjugated verb (gardes), and the que goes directly in front of the specific thing being restricted. Here, it restricts une possession, meaning you keep only that.

Why is it "qu'une" instead of "que une"?

The word que always drops its final -e and adds an apostrophe when the following word begins with a vowel or a mute H. Since une starts with a vowel, que une becomes qu'une.

Why doesn't "une" change to "de" here, like it usually does in negative sentences?

This is a very common trap! In standard negative sentences with ne ... pas, indefinite articles (un, une, des) change to de. But because ne ... que is a restrictive negation (it means "only," not "not"), the rule doesn't apply. The article doesn't change, so it stays qu'une possession.

Could I just use "seulement" instead of "ne... que" for this sentence?

Yes, you could say Tu gardes seulement une possession. Both mean the exact same thing, but ne ... que is extremely common in everyday French, so you need to be very comfortable hearing and using it.