Particles I: Wa and Ga
Two of the most used particles in Japanese are wa and ga. Both are used as subject indicators. However, there are some difference between the two. Often, wa is referred to as the topic particle and ga is referred to as the subject particle. There is a very complex distinction between the two, which boils down to this: wa is used to indicate the general topic and emphasize what comes after it, while ga indicates and emphasizes the general topic. See the examples below.
Tegami wa kimashita.
The mail came.
Speaking of the mail, it came.
The mail came.
Speaking of the mail, it came.
Tegami ga kimashita.
The mail came.
The mail came.
Ga can also be used to point out subjects within a larger topic:
Ayumi ga benkyou gakkou wa tsumaranai desu.
The school that Ayumi goes to is boring.
The school that Ayumi goes to is boring.