ju

Ju is the opening move of a two-part construction and never appears without its partner: ju pli X, des pli Y, the more X, the more Y. The ju clause states the condition, the des clause the consequence, and each half carries pli or malpli. English recycles the same "the" for both halves, which is why learners keep writing ju pli... ju pli; that is simply wrong, and the fix is to remember the pair as one unit. A comma between the halves is standard. The full machinery of comparing things lives in the comparisons grammar article.