what the Different between wa（は） and "ga（が） ?
I am confuse how to use the word wa（は） and "ga（が） in a sentence. Hope someone can explain to me.

Question

what the Different between  wa（は） and "ga（が） ?
I am confuse how to use the word  wa（は） and "ga（が） in a sentence. Hope someone can explain to me.

Answer

basically "wa"　refers to someone or something for the first time.  like indefinite article a.
kore wa pen desu. = This is a pen. 
"ga" refers to someone or something already mentioned. like the.
kore ga sakki watashi ga itta pen desn. = This is the pen which I mentioned just a minute ago.

Of course there are lots of exceptions.