This is not an "if...then" statement, so -chugh is not correct here. -chugh is not used for all occurrence of "if" in English, only for those that are part of an "if...then" statement. A more direct/literal translation of this sentence would be, "Imagine that you can speak Klingon." In English, these kinds of hypothetical sentences are usually worded in the subjuctive as, "Imagine if it were true," or "Imagine if you could..." This sentence occurs far enough into the course that we are trying to teach you to let go of direct and literal translations and think about the meaning behind the phrases to match them correctly. I've added an explanation of this to the Tips & Notes of this unit.