Usage of "that"

[newy]

Even a grammar freak like me needs some help every now and then.  Which is correct (or is there a correct)?

"...the essential skill of speaking clearly and gently raising my voice when I knew something was wrong."

"...the essential skill of speaking clearly and gently raising my voice when I knew that something was wrong."

And also, if someone could point me to a rule on the usage of "that", it would be most helpful.  Thanks in advance.

[egg]

I ran into this in my apps, too. From the ever-awesome bartleby.com:

You can omit that in a relative clause when the subject of the clause is different from the word or phrase the clause refers to. Thus, you can say either the book that I was reading or the book I was reading. You can also omit that when it introduces a subordinate clause: I think we should try again. You should not omit that, however, when the subordinate clause begins with an adverbial phrase or anything other than the subject: She said that under no circumstances would she allow us to skip the meeting.

http://www.bartleby.com/64/C001/062.html

So either is fine in this case, though I'd use the first one.

[newy]

Thanks!

[seankenn]

My high school english teacher said to always drop the use of that.
She was a grammar nut.  So I believe her.  So I try to avoid using it at all costs.  But like the above poster said both are okay, but the first one would be a stronger sentence.