Negatives are easy; prepend “nen-” to the verb, before the argument prefix. So nenpéergòl, he doesn’t see her.
Yes/no questions use the interrogative particle “ash”: Ash péergòl?, does he see her? Note that negative yes/no questions are not handled quite the same way as in English; if you didn’t go, the correct answer to Ash kall nensúebwàt?, Didn’t you go? is “Edh”–Yes, you’re correct, I did not go. Conversely, if you did go, the right answer would be “Nen”, meaning “No, I did go.” It’s probably easier if you think of the question as meaning “Did you not go?”.
(There’s also “kall”, the second-person pronoun. Like the first-person “doz”, it indicates that the subject of the sentence is not in the default third person.)
Informational questions also begin with the interrogative ash. The part you want to know is replaced by an appropriate word and assumed to be the object of the sentence. Ash monid péergòl? is “Who (feminine, singular) does he see?” (or perhaps “He sees who?”); if you want “Who sees her?” you’ve got to phrase it as Ash monid záathergal?. Note that with “za” the being doing the seeing is assumed to be epicene and singular, and that the literal translation is closer to “She is being seen by who?”
monid: who; thokis: what (animals); poliy: what (things); wodhiv: where; zofill: when; gozhib: why; lorin: how