I read about this topic last week. Actually I just had a quick flip through the green book(Language Description book) on that day. I found it quite interesting, so I decided to share it here. I don't think I'll read this book if there's no examination for this subject.  >.< 
Interjections are used in our daily conversation. I think I use a lot of 'hmm' .... :) We need to learn interjections and how to use them correctly. This green book is really useful and has more details about this topic. It's very easy for us to study. How nice if we had 'green book' in every subjects.

Interjection is used to express a sudden emotion.

Examples: Ouch!  I'm so happy!   X
                  Wow! I'm so happy!     /

To express happiness
yipee,hurray,wow

To express sadness
Oh,aww,emm

To express disgust
ermh,huh,disgusting

To express anger
grr,urgh,s**t

To express surprise
wow,amazing

To express shock
ooh

To express pain
ouch,ugh,argh

To express pleasure
emm,alas,bravo


Reference
Norhazian Mat Sapian and Yong Lee Choo(2011). Language Description 1, Selangor: Oxford Fajar Sdn.Bhd.

That's all. ^.^ 11/11/11