In Dutch, we use the apostrophe in the following cases:
To show that a letter has been omitted
||in the evening
||in the winter
(*) des is an old-fashioned Dutch form of "of the", which we do not use anymore. We still use it in some common expressions, but mostly in abbreviated form with an apostrophe. Those of you who know German will recognize the 2nd declension here.
To keep a vowel long when adding -s to a noun
If a noun ends in an unstressed vowel, its plural gets -s at the end. To
keep the long vowel long, we use an apostrophe. Recall, that if a single vowel sits in a syllable that ends in one or more consonants, it is a short vowel).
Not all vowels need an apostrophe to remain long:
The acute accent already tells us that the vowel is long - we do not need to keep it long by adding an apostrophe.
The mute e at the end of schaapje is pronounced as English the. The sound does not change by adding the letter s: the e is still mute.