刚学英语，问一下关于主格人称代词的问题？

你分析的不对。
第一句话，有点错误，应该是likes，因为第三人称单数。
第二句话，it是主格没错，但是dress不是补语，它在第一句话中做Like的宾语。
第三句话，跟第二句话一样，It是代词宾格这个没错，可是dress不是补语。

英语补语
英语补语的作用对象是主语和宾语,具有鲜明的定语性描写或限制性功能,在句法上是不可或缺的。补语是起补充说明作用的成份。最常见的是宾语补足语。名词、动名词、形容词、副词、不定式、现在分词、过去分词都可以在句子中作宾补。
一、主语的补语：它用在系动词后，是句子的一个基本成分。常用主－系－表结构。
1.I saw her with them, at least, I thought it was her.
我看到她和他们在一起，至少，至少我认为是她。（her做宾 语，them做介词宾语，her做主语补语）
2.. -- Who broke the vase? --谁打碎了花瓶？
-- Me. --我。 （me做主语补语= It's me.）
3.John hoped the passenger would be Mary and indeed it was she. （she做主语补语）
约翰希望那位乘客是玛丽，还真是她。
二、宾语的补语
1.不定式(to do)
Father will not allow us to play on the street. 父亲不让我们在街上玩耍。
We believe him to be guilty. 我们相信他是有罪的。
We made him copy the sentence.
He is made to copy the sentence.
I felt my hands tremble.
2.名词
At the meeting we elected him monitor.
I think your brother a clever boy.
3.形容词
What you said made Xiao Wang angry.
I found the classroom empty
4.副词
Please call the students back at once.
He was seen to take his cap off.
5.现在分词
We hear him singing in the hall.
I found him lying in bed, sleeping.
6.过去分词
He saw his face reflected in the water.
I heard it spoken of in the next room.

人称代词的宾格在句子中作宾语或介词宾语，但在口语中也能作主语补语，第一人称在省略句中，还可以作主语，例如：
I saw her with them, at least, I thought it was her.
我看到她和他们在一起，至少我认为是她。(her做宾语，them做介词宾语，her做主语补语)
a. -- Who broke the vase?--谁打碎了花瓶？
b. -- Me. --我。(me做主语补语= It's me.)
说明：在上面两例句中，her和me分别作主语补语。现代英语中多用宾格，在正式