英语简单滴

首先要纠正，女主持人应该是hostess，host是男主持人。
The girl under the tree is a hostess and singer.(singger前不可加a,否则就成两个人了）
The girl under the tree is a hostess,who is also a singer.(用定语丛句补充说明）
The girl under the tree is not only a hostess but also a singer.（不仅是……而且是……）
The girl is a singer as well as a hostess.( 树下的那个姑娘不仅是主持人也是歌唱家）（你那个句子本身是对的，但是，你前面说的是强调singer“她是主持人，也是歌唱家。”而你的这句话是强调host。他不仅是歌唱家而且还是主持人。）

The girl under the tree is not only a host but also a singer.
The girl under the tree is a host as well as a singer也可以。

The girl under the tree who is also a singer is a host.

The girl is a host and a singer who under the tree.

The girl under the tree is not only a host but also a singer .

我认为你那么说也可以。

The girl under the tree is host and She is also a singer