Complex sentence: When an aircraft is moving forward, the leading edge is that part of the wing that first contacts the air-it can be said that e.g. during tailslide, from an aerodynamic point-of-view, the trailing edge becomes the leading edge and vice-versa.
Split into simpler sentences: When an aircraft is moving forward, the leading edge is that part of the wing that first contacts the air. During a tailslide, from an aerodynamic point-of-view, the trailing edge becomes the leading edge and vice-versa.