why do the Christians i know believe that the old testament does not apply to them. it is in their holy book. i dont understand and would like some insight on why this is.
Not entirely true. What you're referring to is the old Mosaic law that applied to Israel and Israel alone. The law came to be because of their moral corruption, and again, this was Jewish law.
That is what does not apply, the legalism of the OT.
To say that the OT somehow doesn't apply is wrong, and some Christians would like to believe this somehow extends to the 10 Commandments or any other teaching of morality and depravity then being "okay" simply because Christ "fulfilled" all law in their mind.
In short, Christians are not under Mosaic/OT law, it would essentially go against everything that Christ came to fulfill. However, the entire OT is not some several hundred page list of laws, it's a small fraction. Some Christians like to entirely ignore the OT and they have that line of thinking. Ex: If you're one to believe the biblical prophecies that Jews would regather from the four corners of the Earth and Israel would be reestablished, you would have to believe in OT prophecy, although there are Christians who oppose Israel simply because "Jesus came and fulfilled the OT!" when that's a half-truth on their part.
Another way to look at it, Jesus came to usher in the new and toss out the old. As was said in Romans and Galatians,
Galatians
13Christ redeemed us from the curse of the Law, having become a curse for us
There's a litany of other passages, but that's the gist of it. There are specifics of what is/was fulfilled, but it does not negate the entire OT teachings that apply to any and all, not just Jews, unlike the Mosaic law, which was just for Israel.
E: What I should be asking though is what do you want addressed specifically? The bible is very complex, and even most Christians who go to church every Sunday to do their "Christian duty" sitting there for an hour (probably sleeping) get a lot of things wrong as well. If there's something specific you'd like to talk about feel free to ask.
E2: One thing I know is the most difficult to grasp is that everything shouldn't be taken literally. Ex: Jesus used the same example as was used in the OT, cutting off of the hand or gouging out of the eye, but that would be entirely false as Jesus never not once preached violence. Another example would be this, do you really think Elijah was taken up by a flying magic horse and chariot? This is where the topic of addressing the times in which things were written, and why most people fail to understand that the bible (a collection of books, bible = library) is more of a
historical document that should be read with respects of the time in which it was written, rather than a modern fantasy novel written by J. K. Rowling.