The Seine is a river in the north of France. It runs through Paris, then through Rouen, and meets the sea in the English Channel near Le Havre. In English we treat it as a place name: we write a capital S, and we almost always say “the Seine,” not just “Seine.” You will meet it in travel talk, in news, and in stories about Paris — walks by the river, bridges over it, boats on it. People use it as a landmark, not as a science word. Though actually, English also has another word that looks the same if you ignore the capital letter: a seine is a fishing net you pull through the water. That is a common noun, and it is not this river. Students (and even spell-checkers) mash the two together because they sit on the same letters. When the S is big and there is a “the” in front, you are in France, on the river that Paris grew up around.