If a vowel contains a dieresis, it forms the beginning of a new syllable. It is used to indicate where we have to divide a [double vowel](#/lesson/SpellingAndPronunciation.10) or [vowel combination](#/lesson/SpellingAndPronunciation.12) into separate syllables. Without the dieresis, a double vowel or vowel combination would make only one syllable.
e-go-is-tisch | egoïstisch | selfish
The _oi_ in _egoistisch_ would be pronounced as the English _oy_. If we give the _i_ a dieresis, as in egoïstisch, the _oï_ is pronounced as the English _ohwee_.
re-eel | reëel | real, realistic
Without the dieresis, we would not know where to divide the sequence of e's into syllables. It could be re-eel, but also ree-el.
sym-pa-thie-en | sympathieën | sympathies, preferences
The same goes for sympathieën. Without the dieresis, we would not know how to choose between sym-pa-thie-en and sym-pa-thi-een.
As you can read in the section about long vowels, the combination ui is pronounced as one long vowel. In the word altruïstisch, we want to pronounce the u and the i as two separate vowels. To establish this, we need to put a dieresis over the i.
without a dieresis (wrong!) | altruistisch | in separate syllables: al-truis-tisch
with a dieresis: | altruïstisch | in separate syllables: al-tru-is-tisch
When we break up a word at the end of a line in such a way that the syllable at the beginning of the second line starts with the dieresis, we do not write the dieresis (it is already clear by the way the word had been divided).
ego- | re- | sympathie- | altru-
istisch | eel | en | istisch
the Flemish quality paper De Tijd and select an article. Now pick a few dozens of words and divide them into syllables.