Vul het formulier in.
Ik wil invullen.
Ik heb ingevuld.
Fill in the form.
Yes. invullen splits into in + vullen. In a main clause the prefix goes to the end: Vul het formulier in.
The perfect tense is "Ik heb ingevuld" — the -ge- slots between the prefix and the stem.
invullen means "to fill in". Example: Vul het formulier in. (Fill in the form.)
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