这 四个英语句子对吗:I'm cat. I'm a cat. It's letter w It's a letter W

I'm cat. 错
I'm a cat. 对
It's letter w 错
It's a letter W 错

It's the letter W.

I'm cat. 错
I'm a cat. 对
It's letter w. 错
It's a letter W.对

I'm cat.错
I'm a cat.对，要有冠词修饰
It's letter w ，对，
It's a letter W，错

I am cat. wrong.
I am a cat. right
It is letter W. right
It is a letter W. wrong
因为只有 26个英文字母，都是特指的，不能用a 修饰