in+时间 有两成意思
在多少时间内 【用于过去时态】如：
He finished his homework in half an hour yesterday.
在多少时间后 【用于将来时态】如：
His father will come back in an hour.
for+时间 ....多少时间 【可用于各种时态】如：
He will stay there for a week
I have learnt English for five years.
Jack stayed in China for three years.
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