ground

1.“一块地”不可说a ground，该说a piece of ground。复数形式不指“多块地”，却指“一块围地”，通常在屋的四周，有草地、矮树丛等，如school grounds和temple grounds。grounds前面用in，不用on。

2.on the ground that...和on the grounds that...相同，都作“以…为理由”解。例如：He was severely criticized on the ground(s) that he did not work hard. on the ground of（以…为理由）里不可用复数形式。on...grounds（以…的理由）里不可用单数形式ground，如该说on public grounds和on humanitarian grounds。