"Ik ontwerp de broek."
Translation:I design the pants.
The "Ik ontwerp de broeken" variant is also correct, but it has the different meaning. "Ik ontwerp de broek" - I design the pants (I design the (singular) pair of pants), "Ik ontwerp de broeken" - I design the pants (I design the pairs of pants (plural)). The Dutch language has "een broek" - pants (a pair of pants) and "broeken" - pants (pairs of pants), while in English language we use "pants" word for both, singular and plural, but if you want to say more accurate amount of pants in English, you use the "pair" word to make sure you mean singular or plural. :)