Learn Dutch: Mijn zus heeft die foto's onlangs gevonden. - My sister found those photos recently.

It is the past participle of the verb vinden (to find). Just like "find" becomes "found" in English, vinden is an irregular verb in Dutch. Instead of adding a standard -d or -t at the end, the middle vowel changes from "i" to "o", making it gevonden.

Why is gevonden placed at the very end of the sentence?

In Dutch, when you talk about the past using a helper verb (heeft) and a past participle (gevonden), the helper verb takes the normal second position in the sentence. The past participle then gets pushed all the way to the very end, after "those photos recently".

Why is there an apostrophe in foto's?

In Dutch, nouns that end in a single open vowel (like a, i, o, u, or y) take an apostrophe plus "s" ('s) to become plural. This is a spelling trick to keep the "o" sound long. If we just wrote "fotos", the rules of Dutch pronunciation would make the "o" sound short!

Why do we use die to mean "those" here?

Die is the demonstrative word you use to point at plural nouns, meaning "those" (or "these"). Because foto's is plural, you must use die. You only use the other word, dat, for singular "het" words.