A distinction can be made between open parts of speech and closed parts of speech.

Classes of open parts of speech are expandable: new words can be added to this group. New adjectives, adverbs, nouns and verbs can enter the language. You can tell when these new members of the group have been successfully integrated: they follow the grammatical rules of Dutch.

Dutch has always 'borrowed' words from other languages, most noticeably from Latin, French, English and German. English is still a major influence today. Just think of all the new words that have become part of everyday language since the introduction of the computer or mobile phone, for instance. Especially the younger generations use English words in their speech and, to a lesser extent, writing.

More recently, however, young Dutch speakers in the bigger cities also increasingly use words taken from the languages of the ethnic minorities who have large representations in these cities. Moroccan-Arabic, Berber, Turkish and Sranan-Tongo have all been reported to influence Northern Dutch youth language (see: J. van den Braak @Het verschijnsel straattaal).

The second category is called closed because new words are generally not added to this group. It is very uncommon for a new numeral to be 'invented', for instance. But even here you should not underestimate the creativity of language users!