---I'm sorry,but it's my seat.

---           I didn't know it's yours.

What if

I beg your pardon

That's all right

What for

考查交际用语。句意：---对不起，这是我的座位。---请原谅。我不知道这是你的。A. What if如果.....将会怎么样；B. I beg your pardon. 对不起，请原谅；C.That's all right没关系；D. What for为什么。根据"Idon't know it's yours."可知坐了别人的座位，请求别人原谅。故选B项。

Only then_______ how much damage had been caused.

she realized

she had realized

had she realized

did she realize

“only＋状语”位于句首，整个句子用不完全倒装．此外，由then可确定用过去时态．

Mary never does any reading in the evening, ______.

so does John 

John does too

John does not to

neither does John

主句用never，后面的倒装句相应的用nor或者neither，用完全倒装形式表示后者与前者情况相似。

Small sailboats can easily turn over in the water ________.

if not managed carefully

if are not managed carefully

unless not managed carefully

unless are not managed carefully

How I wish every family         a large house with a beautiful garden!

has

will have

had

had had

考查虚拟语气。句意：我多么希望每个家庭都有一个带着美丽花园的大房子！此句是wish 引导的从句，从句要用虚拟语气，根据wish 可知是与现在事实相反，从句一般用过去时。故选c项。

Perhaps they have experienced something similar and do not want you to           the same pain.

go over

go through

go around

go on

考查动词短语辨析。句意：也许他们有过类似的经历，不希望你经历同样的痛苦。A. go over复习，重温；B. go through经受，通过；C. go around 四处走动，流传；D. go on继续。根据the same pain可知，此处是“经历，经受”的意思。故选B项。