Just saw some stuff that I thought i'd correct. Hope people find this helpful.
Nani!?
Sempai
Doushite
E-tatata!
Sempai = Senpai. m cannot be without a verb after it. n's can also sound like m before some consanants, so this is understandable.
as someone else said, I think by "etatata", you mean the adj itai. hurts, painful, whatever.
ike saying "itadakimasu" before eating and "ittedekimasu" before leaving the house
ittedekimasu doesn't really make sense. Gone having being able. What you thinking is itte kimasu. Gone having come. Meaning I'll go (and then come back). Itadakimasu is also an honorific verb to eat.
okari
okawari? okawari = seconds (like food). okari as far as I know doesn't exist..
In real life I have observed that they tend to nod their head deeply when greeting or saying their farewells (sayonara, ja mata).
I don't hear sayounara that much. Friends tend to say something like "ja ne" or "mata ashita". If you're talking to your teacher, I usually say something like "mata <day> ni aimashou ne?" which means "let's meet again on <day>, right?" It's a handy phrase. If you're saying goodbye, you could also say "shitsurei shimasu" or "osaki ni shitsurei shimasu" (if they are still staying in the classroom).
For 'huh?' usually I just hear, 'eeee?' (pronounce it like a lengthened short "e", like the /e/ in "head").
I think you mean "heee". Most students cant say it with a straight face. It usually goes from low to high in voice intonation (or whatever).
you can say "ee" for huh/what, but it's not usually drawn out long. Just a short simple ee?
as for de mo, there are lots of different ways to say but, however. Just keep that in mind
Also, I've learned a couple interjections, such as "wai" and "kawaii" ... and the extreme broadness of the word "hai" (Notice how it's used for nearly every affirmative answer?)
I hear "ee" and "n" just as often. it all matters who you are talking to.
Osuwari(sit)
Suwaru is better for standard use.
</nitpick>Yeah i'm nitpicky. sorry. And just a note... If you don't know the exact meaning and politeness of a word, you really shouldn't use it except with friends. When watching anime, you pick up a lot of words that seem okay to use, but they aren't. Some people might not care, but it's better to be more cautious than to have people think you're being rude.
As far as the whole ou, ei, thing. I always use the hiragana to romanji type spelling (ex: arigatou, ohayou, etc). It makes it easier for people to learn the correct way to spell it, assuming you're not learning the kanji.