Complex sentence: The diaeresis (Dutch: "trema") is used to mark vowels that are pronounced separately when involving a pre- or suffix, whereas a hyphen is used when this problem occurs in compound words.
Split into simpler sentences: The diaeresis (Dutch: "trema") is used to mark vowels that are pronounced separately when involving a pre- or suffix. Whereas a hyphen is used when this problem occurs in compound words.