Problems with posts

I have a continuing issue with the posting section. This has been ongoing since I started with WordPress a few months back. It will “Fix” code to the point where code dissapears from my post. An example:

I created a form in a post. I wanted to take out the form because it wasn’t working right. So I commented it out with ! –. When I went back into the code, it fully removed the </ form> part.

This isn’t the first time. Every time I put in code that’s not br or p, it freaks out.

Is there something that can be done about this?

Thanks.