Christmas Day was coming.Father and Son went to get a Christ

1 Christmas Day was coming 过去将来时（进行时中动词如果是瞬间动词可以用来表示将来）
2 Father and Son went to get a Christmas tree一句中
Father and Son 作主语
went 谓语动词
to get a Christmas tree目的状语
3 go to 后面接的不是从句.从句必须具备有主语,但go to 后面没有.
I was surprised to learn that he was taking drugs.这句中surprise是形容词“惊奇的”,非动词
to learn that he was taking drugs是不定式作原因状语,说的是惊奇的原因；
They were surprised to find the animals at their gate一句中
they是主语
was是系词
surprised是表语
to find the animals at their gate是原因状语

Christmas Day was coming.的时态是过去进行时
Father and Son went to get a Christmas tree.中，Father and Son是主语，went是谓语，to get a Christmas tree.是一个以to开头的宾语从句，用来做went的宾语。
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is coming才符合逻辑，但是用was也能讲得通！ 第二句，很简单的一句话，有啥不懂得？