[RadioKid]

Even native Japanese people sometime discuss about "HA" and "GA" and will not agree each other every time.

As for me, I agree with Chryuop (Watashi HA Chryuop-san NI Sansei Shimasu).

WO(or O) definitely indicates direct object.
GA indicates subject as a principle and sometime indicate object as rare cases (used with like, love or else).
HA is indicater of main topic and should not used more than once in a sentense while GA can be used many times in sub-sentense to indicate subjects in sub-sentenses.

HA indicates sometime subject and object other time. And also HA implies some limitation. For example, if someone says "SAKANA HA KATTE KITA YO" (I bought fish) implies he/she forget buying something other than fish.

When GA is used to indicate the subject, the sentense is just a fact and corries no intention of the talker. It may sound strange for western people because English always tells the fact. (Sorry, I can not continue because it is too difficult to explain in detail)

For example, "Watachi GA Yari mashita" (I did) is just a fact and it is enough as an answer for the question "who did it?". If you want to tell your opnion, you should say "Watashi HA KOU Omoi masu" (I think that...).

As as extreme example which is well known by Japanese People, "Watashi HA UNAGI Da" which means "I will order Eel (following to other people's order)" instead of "I am an eel".

I hope these help you.