a knapsack
knapsack 是可数名词，单数形式前一般要用冠词 a/an。
He bought a knapsack before the trip.
旅行前他买了一个背包。
可选值集合；一组可从中选择子集的值。；一种背在背部或肩上的带子包
一组可从中选择子集的值。
A set of values from which a subset is chosen.
一种背在背部或肩上的带子包
a bag carried by a strap on your back or shoulder
He packed his knapsack with food and water for the hike.
他背包里装满了徒步旅行用的食物和水。
用法提示：knapsack 作为名词，表示背包。
The old knapsack was frayed from years of use.
这个旧背包因多年使用而磨损了。
用法提示：knapsack 作为名词，指背包。
She slung her knapsack over one shoulder and set off.
她把背包挎在肩上，出发了。
用法提示：knapsack 作为名词，表示背包。
knapsack 是可数名词，单数形式前一般要用冠词 a/an。
He bought a knapsack before the trip.
旅行前他买了一个背包。
可用 my、your、his、her 等物主代词直接修饰，表示所属关系。
She hung her knapsack on the back of the chair.
她把背包挂在了椅背上。
复数形式为 knapsacks，指多个人各自的背包。
The hikers dropped their knapsacks on the ground.
徒步旅行者把背包扔在地上。
表示“在背包里”用介词 in，不用 on。
He kept his lunch in a knapsack.
他把午餐放在一个背包里。
grammar · medium
易错句子He put his lunch in knapsack.
正确表达He put his lunch in a knapsack.
knapsack 是可数名词，单数形式在句中泛指时必须带冠词，所以要用 a knapsack；汉语名词常可裸用，这是母语迁移导致的漏冠词。
grammar · medium
易错句子They bought two knapsack for the trip.
正确表达They bought two knapsacks for the trip.
knapsack 是可数名词，前面有数词 two 时须用复数形式 knapsacks；汉语名词没有单复数形态变化，学习者容易忘记加 -s。
grammar · medium
易错句子She carried a knapsack in her back.
正确表达She carried a knapsack on her back.
表示“背在背上”英语固定用介词 on（on one's back），in 指在物体内部；汉语“在背上”容易被直译成 in。
所指的包几乎相同，但 backpack 是现代英语的通用词，日常、上学、旅行都能用，还可作动词（to backpack 徒步旅行）；knapsack 偏旧式、书面，多指帆布小背包。
主要用于英式英语，指登山或长途旅行时背的较大帆布包；knapsack 通常更小、更旧式，日常口语中已少用。
satchel 通常只有一条背带，斜挎在一侧肩上，多为学生书包或皮包；knapsack 是用两条带子背在背上的包。
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