"I saw a tall Klingon."
Translation:tlhIngan woch vIlegh.
The verb prefixes jI- and vI- (capital i!) are described in the tips and notes for the first unit ( https://www.duolingo.com/skill/kl/Useful-phrases/tips-and-notes ) -- if you haven't been reading the tips and notes so far, now would be a great time to start.
On the website https://www.duolingo.com/ , the tips and notes for a unit are available by clicking on the unit and then on the little lightbulb icon:
The tips and notes are not available (for most users?) in mobile apps, and I wouldn't recommend using those for learning Klingon since you'll be missing out on all the grammar explanations. (They may be useful for repeating units that you have already learned on the website.)
If, after reading the tips and notes, you still have questions, or if something seems contradictory or confusing, please feel free to post about that in the discussion forum or on an appropriate sentence discussion page.