Question

It was two years ago __I joined the army A．since B.if c.when d.while

Answer 1

应该是 that.
这个问题只有从 that 和 when 中考虑.因为本身只可能这两个选择.that 的话是强调句,when 的话是定于从句.
然而,判断这两者的办法是：先把“it was”和空格去掉,看一下剩余部分能否组成一个正确的句子,比如这句,去掉后变成：Two years ago I joined the army.
本来是 I joined the army two years ago.为什么倒过来了呢,其实就是为了强调的需要.因此,去掉后的这句话是对的.
如果是对的,那么就是强调句.就用that .
如果不对,就是定语从句.就用when .
这里再举一个例子：
It was one o'clock _____ I went home.
A that B when
填哪一个呢?
去掉 It was 和空格,变成：One o'clock I went home.这就不对了.因为正确的句子应该是：At one o'clock I went home.显然缺了介词.
那就是定语从句,选 when .
至于你另外几个选项,since 可以这么用：I have been in the army since two years ago.
If 引导条件句,或“是否”,这里不能用,你应该知道吧.
while 届时为“当”的时候,后面要接进行时态的动词,解释为“然而”的时候,这句句子不适用,因此不能选.