Ahhh, I'm sorry for posting so many Help topics guys, but I really need help again

Usually when writing an email or just writing text on a webpage, like now, I can press Tab to move along a bit (instead of spacebar, y'know what I mean).

However if I press Tab now it just goes down to another item on the page, for example the checkboxes below where I am posting, then the submit thread button and so on.

Can anyone help? Sorry I'm so clueless (you wouldn't believe I studied IT would you )