来看几个易混淆的语法句 是否要加and连词

以下是我随便编的一个句式

发觉G考语法中经常碰到类似状况

请帮忙看看每一种情况应不应该加and(连词)

1.I have several books, (and) one of them has a profound function.

2.I have several books, (and) each of them has a profound function.

此句中的each是作为代词的.正确答案是应该加and才对.

3.I have several books, (and) they have profound function.

4.My several books are expensive and (they) have profound function 此句中they是不是加上就错啦?