(1) I cannot think why you said so. 

我想象不出你为什么会那样说。

主语：I

谓语：can't  think

宾语从句:why you said so

解析：以上为宾语从句

I cannot think why you did it!
我不能理解你为何要干那样的事!

(2) You can’t think how happy I am to see you again.

你无法想象我再见到你是多么地高兴。

主语：You

谓语：can't think

宾语从句:how happy I am to see you again

解析：以上也是宾语从句

She was thinking how cold the room was.
她在想那间屋子可真够冷的。

She was thinking how strange the children were.
她在想,那些孩子可真怪。

I was just thinking what a long way it was.
我刚才还在琢磨着这条路多长啊。

I cannot think how I came to do it.
我想不到我怎么会干起这样的事来。

I couldn't think where she'd gone.
我想象不出来她到哪里去了。

I can't think what his name is.
我想不起来他叫什么名字。

I cannot think where I put it.
我记不得我把它放在哪儿了。

think作为及物动词，后接名词用法

They thought one thought.
他们想法一致。

Think happy thoughts and you'll feel better.
想一些令人高兴的事情你就会觉得好一些。

They ought to be encouraged to think their own thoughts.
应该鼓励他们独立思考。

What makes you think that?
你怎么想到那个?

Who'd have thought such a thing?
谁能料到有这样的事呢?

If you want to make money you've got to think money.
如果你想赚钱,脑子里就得想钱。

当think接宾语从句时应该看为及物动词。英语中很多动词具备及物不及物双重功能的。如：Think the happiest thought you can think. I'll think it over. 这不都是及物用法么。关于你的第二句，你没必要研究这么细，可以理解为感叹句how happy I am to see you again变成think后的宾语从句即可。学英语，句子成分分析不是目的，是手段，只要知道如何正确使用即可。这就是为什么英语考试中，一般都不考句子成分分析。当然，我们是语法网站，另当别论。我仅是从学英语角度来说的。