about Verb tense

in fact. i knew a girl on internet many day ago.

1. I know the girl on the internet . This is correct.  But you might prefer to say a girl, not the girl.
I know the girl on the internet since many days ago. As soon as you add "since" you can't use present tense.  Say "I met the girl on the internet many days ago."
2. I knew the girl on the internet many days ago .
3. I have known the girl on the internet before for many days ago. We don't use before or since in the same phrase as ago, but we can use them together.  "I have known the girl since Friday." "I knew the girl before today." "I have known the girl since before Sunday."

Present tense in grammar . I have read a point that is to Description Facts.
I am not certain The meaning .

事实是。我在几前在互联网上认识了一个女孩。
我可以怎么使用不同的时态来说明这件事.不知道我表达的方式是否正确.

在文法简单现在式中。我读到了一点那就是"说明事实。 "
我不能肯定我的用法是否正确。