どうした？ (doushita?) = What Happened?
なに？ (nani?) = What? 
質問 (shitsumon) = Question.
だれ? (dare) = Who?
いつ? (itsu) = When?

どうして？ (doushite?) = Why?
なぜ? (naze?) = Why?
なんで? (nande?) = Why?

If you want to ask why, you can use naze or doushite or nande.
If you want to ask how, you can use doushite or nande, but cannot use naze. 
Doushite and nande are used not only to ask why but to ask how.

手洗い (tearai) = Bathroom. - when you need to go to do your business
助けて (tasukete) = HELP.
わかりません (wakarimasen) = I don’t understand. 
やめて！ (yamete!) = Stop it!
英語 (eigo) = English
警察 (keisatsu) = Police. - If something really bad happens, these are the guys you’re going to want to ask for.
危ない (abunai) = Dangerous. -  You’ll see this on signs and stuff too. If someone says “abunai” to you, you should probably watch your step.
危険 (kiken) = Peril, Hazard. 

These are some words and greetings that you can say without being embarrassed.

English word                        Romaji                           Japanese

Hi/Hello                               Konichiwa                      こんにちわ

How are you?                     Genki desuka?                 元気ですか？

What are you doing?            Nani shite iru no?               何しているの？



Slangs:

English                                Romagi                        Japanese

Um…                                    Ano…                           あの。。

Stupid                                 Aho              あほ (used in kansai areas)

Idiot                                   Baka                            ばか

I see                                   Ah sou                あぁ、そう (informal)