Take my fifth grade teacher, for example. Here's a picture of my classmates and me at Foster Park Elementary. (See, you'd probably say "me and my classmates," or "my classmates and I," am I right? Bad, bad.) That's me right there in the middle, second row. Our teacher was Mrs. Mazyck (pronounced "ma-ZEKE"). She was something else. She drilled us over and over again on rules of English grammar. She would play these records and we'd have to repeat what we heard - endlessly. But I must say, it worked. I learned things that came in handy later on in high school, college, and now in my life as a preacher.
So from time to time I'll share a bit of my knowledge and give a "grammar lesson of the day" here in my blog.
For my first lesson, class, let's talk about something you're never supposed to say: "The reason is because...." That's incorrect, you see. The RIGHT thing to say is "The reason is that...."
Suppose someone asks you why you're carrying an umbrella. You could say, "I'm carrying an umbrella because it looks like rain." Or you could say, "The reason is that it looks like rain." But you shouldn't say, "The reason is because it looks like rain." That'd be redundant.
And that's why it's bad grammar.