Ik begrijp het niet.
English: I do not understand it.
In Dutch, 'ontrennbare werkwoorden' are verbs with a prefix that cannot be separated from the verb. These prefixes always stay attached, even when you change the tense or make questions.
Use these verbs when the action has a fixed meaning with the prefix. The prefix changes the meaning of the verb, and you never split it from the verb, even in sentences with auxiliary verbs or in questions.
Ik begrijp het niet.
English: I do not understand it.
Zij heeft het verhaal herhaald.
English: She repeated the story.
Wij vergeten vaak de tijd.
English: We often forget the time.
Hij ontkent zijn fout.
English: He denies his mistake.
Jullie bezoeken de stad.
English: You (plural) visit the city.