The difference of “ha” and “ga” is supposed to be one of the fundamental and complex issues on Japanese language. You can apply the explanation below in most cases, but I bet you’ll meet a lot of exceptions inexplicable with this. As of your understanding, it was quite close but subtly incorrect. Here it is. This is my friend. : Kare HA watashi no tomodachi desu. This is the friend of mine. : Kare GA watashi no tomodach desu. His name is Jack. : Kare no namae HA Jack desu. The name of his is Jack. : Kare no namae GA Jack desu. You see the differences. The first and third sentences are plain, and on the other hand, the second one emphasizes on THE FRIEND, which implies that “this is the friend of mine, not the guy over there.” The forth one emphasizes on THE NAME, implying that “the name of his is Jack. The guy over there is not Jack. He is Alex.” Watashi ha tabeta. : I ate. (stress on ate / Did you eat? I ate.) Watashi ga tabeta. : I ate. (stress on I / Who ate? I ate.)