"I have an interesting book."
Translation:paq Daj vIghaj.
Klingon verb prefixes indicate not only the subject of a verb but also the object, so you usually have to use a different prefix when a verb has no object compared to when a verb does have an object.
See the "Verbs" section of the tips and notes for the first unit: https://www.duolingo.com/skill/kl/Useful-phrases/tips-and-notes
If you're using the website www.duolingo.com to learn, the tips and notes for a unit are accessible by clicking on the lightbulb after selecting the unit:
If you're using a mobile app to learn Klingon, you probably don't have access to tips and notes. I recommend not using a mobile app to learn new material (though they may be useful to review previous units where you have already read the tips and notes).
If, after reading the grammatical notes, there is still something that is unclear, please continue to ask, either in a top-level forum post in the "Klingon" section ( https://forum.duolingo.com/topic/953 ) or on an appropriate sentence discussion.