fall downMeans to fall down or fall into a lower place. The ไป indicates movement away or into another location; do not confuse ตกลง, which can mean “to agree.”
TH: อย่าเดินใกล้ขอบนะ เดี๋ยวจะตกลงไป
EN: Don't walk so close to the edge, or you'll fall down.