を (path of motion)
grammar79 sentences
The particle を, normally a direct-object marker, also marks the path, route, or area traversed by a motion verb — 道を歩く ('walk along the road'), 公園を散歩する ('stroll through the park'), 橋を渡る ('cross the bridge'), 空を飛ぶ ('fly through the sky'), 川を渡る ('cross the river'), 川を泳ぐ ('swim across the river'). Not a direct object — a path/space marker. Distinct from the motion-source を that marks the place being left (飛行場を飛び立つ = 'take off from the airport').