So am I 和 So do I 在意思上没有区别， 只是“系动词”或“动词”要和上一句的“系动词”或“动词”一致而已。
如：He made it. Made是动词（do), 故要用“do”的适当形式， 即： So did I. 再如： He is mad.
Is是系动词（be), 也要用“be”的适当形式， 即： So am I.
而So I am 和 So I do同上面的“So am I, So do I”有意义上的不同； 但就其动词或系动词的前后一致而言， 是相同的。
So +助动词/动词BE/ 情态动词等+主语”主要用来说明前面所说的情况也同样适用于后面的人或物，[意为“……也一样”]。该结构中的助动词 do 根据前文的情况也可换成 is, am, are, was, were,does, did, can, could，have/has /had (完成时态中HAVE是助动词) 等。

如：He passed the exam, and so did I. 
=He passed the exam, and I passed the exam, too.

求一首歌,男生唱的,歌词里有许多反反复复的i do,谢谢

I DO Notting Hill诺丁山

I do I do
I do I do
Ooh
All I am
All I'll be
Everything in this world
All that I'll ever need
Is in your eyes
Shining at me
When you smile I can feel
All my passion unfolding
Your hand brushes mine
And a thousand sensations
Seduce me
'Cause I
I do (cherish you)
For the rest of my life
You don't have to think twice
I will love you still
From the depths of my soul
It's beyond my control
I've waited so long to say this to you
If you're asking do I love you this much
I do
Ooh,Baby
Ooh
In my world
Before you
I lived outside my emotions
Didn't know where I was going
Until that day
I found you
How you opened my life
To a new paradise
In a world torn by change
Still with all of my heart
Till my dying day
I do (cherish you)
For the rest of my life
You don't have to think twice
I will love you still
From the depths of my soul
It's beyond my control
I've waited so long to say this to you
If you're asking do I love you this much
Yes,I do
I do
If you're asking do I love you this much
Baby
I do (cherish you)
From the depths of my soul
It's beyond my control
I've waited so long to say this to you
If you're asking do I love you this much
Baby,I do
I, I, I do