（1）The ground must be just right—neither too wet nor too dry.
(2) A lot of good land has gone with them, leaving only sand.
(3) Knock a long, strong stick into the earth next to the hole.
ground, land 和 earth 都作“土地”解, 它们有区别吗?
广义地说,ground 是指从具有一定面积的角度来考虑的“大地表面”“地面”。另外,ground 还作“土壤”解:
We sat on the ground to eat our picnic. 我们坐在地上吃野餐。
与海洋河流相对而言的地面叫 land(陆地、大地):
After two weeks in a boat, we were happy to see land. 我们坐了两个星期的船以后，看见陆地非常高兴。
句(2)中的 land 则另有含义:地皮或农业用地(ground used for farming)(无复数):
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They have bought some land and they are going to build a house on it. 他们买了些地，打算盖一所房子。
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There is good land here for growing corn. 这儿有一块适于种植谷物的好地。
earth 除作“地球”解之外，还作相对于天空的“地面”解或作供植物生长的“土壤”“泥土”解：
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The balloon burst and fell to earth. 气球破裂而落到地上。
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Put the earth back in the hole again. 再把土放回坑里去。