<input type="text" name="email:required" class="formfield [[!+fi.error.email:notempty=`error`]]" id="email" value="[[!+fi.email]]" /> <span class="error">[[!+fi.error.email]]</span>
<input type="checkbox" name="informatie" id="informatie" value="ja" [[!+fi.informatie:FormItIsChecked=`checked`]] />
This question has been answered by Jako. See the first response.
<input type="checkbox" name="informatie[]" id="informatie" value="ja" [[!+fi.informatie:FormItIsChecked=`checked`]] />
<input type="hidden" name="informatie" value="nothing" /> <input type="checkbox" name="informatie[]" id="informatie" value="ja" [[!+fi.informatie:FormItIsChecked=`checked`]] />