Blowout tube question...

Beginner brewer with question about closed fermentation in a 5 gallon carboy. Once the wort is transfered into the carboy and I place the hose in the opening, how can I be sure that no air gets into the wort through the tube...

I'm confused about this because my book says that after the initial activity subsides I should replace the hose with a fermentaion lock, which allows gases to escape, but no air to come in (which makes me think that air is BAD). So how would I stop air coming in during the initial fermentation and blow off?

Maybe a stupid question, but I'm new. Thanks

Put the end of the tube in a glass of water.

If you get vigorous fermentation and your carboy is fairly full you may want to place the end of the hose in a half filled bucket of water. That way if you get a lot of blow-off it won't spill out and make a mess.
Also, I use a diluted sanitizer in the water I place in the bucket.