ein vs eine vs einen

Could you please kindly help explain using the below ein / eine / einen ?
How to use them in the various situation ?
ein Taxi
eine Pause
einen Kaffee / einen Tee

Thanks

Hi Leepboon,

Male and neutral gender nouns have "ein" whereas female gender nouns have "eine" as article. "Einen" is the Accusative case of "ein".
Have a look at Grammar lesson 6.2 for more information about the Accusative case.
In spoken German you will often hear "ein Kaffee/Tee" instead "einen" because it is very common to swallow the ending. 
I hope that helps. Let me know if you have further questions.

Lieben Gruss

Paul