How to use a question word in a statement that isn't a question?
"If you have an English speaking friend" should be translated to 如果你有一个讲英语的朋友. 

Here, 能讲英语的朋友 a friend who can speak English  is much different from 讲英语的朋友 an English speaking friend, since there is a 能 which literally means can.

"I don't know when I should go" should be translated to 我不知道我应什么时候去.

Notice that 我不认识 usually means I don't know (who he/she is)

I'll try to both directly and natively translate your six sentences:


“She's the one who owns this car" 她是（这辆）车的主人
"He told me what you said" 他告诉了我你说了什么 他告诉了我你所说的话
"I will decide when I leave" 我将决定我什么时候离开 我将决定我离开的时间
"He will tell you where to go" 他将会告诉你（应该）去哪
"This is why I arrive early everyday" 这就是为什么我每天早到 这就是我每天早到的原因
"This is how I use my phone" 这就是我用自己手机的方式 我就是**这么**（accent）用手机的


Attributive clause or other forms of clauses exist in Chinese but are much different from English, they could be more implicit, differently structured. You can read this article: How to Translate English Attributive Clauses
As a native Chinese speaker, I did not learn much Chinese grammar, so I suggest you find more in a textbook.