Can I type a "word" without automatically adding it to the user dictionary?

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Sorry for the noob question:
Whenever I type something that is not in my user dictionary (such as an acronym or name) it will change the word or give me the option to touch the left-most suggestion which is the actual word and add it to the dictionary. I understand this... but sometimes I just want to type something that will not likely ever be used again, and I don't want it added to the user dictionary. Can I do this without it automatically saving the new word?
Also, I'm a little confused on the difference between word prediction, word completion, and spell correction. Can someone explain specifically what each one is doing?

if you tap on the text entry box, it will cancel prediction for that word and just leave what you've got without adding to the user dictionary.

As for your second question, I thought they were all aspects of word prediction... i.e. it is predicting that you are going to type this completed word, or it is predicting that you meant to spell it this way... all parts of XT9