Identifying the unknown

「が」 identifier particle

With the 「は」 topic particle, you have to know what you want to talk about ahead of time. Obviously this will not always be the case. For example, if you wanted to know what kind of food somebody liked, it would be impossible to ask if each kind was his/her favorite using the topic particle saying "as for this" and "as for that". That is what the 「が」 particle is for: to identify or seek to identify an unknown.

• ピザは、好きですか？
  As for pizza, do (you) like it?
• どんなピザが好きですか？
  What kind of pizza do (like)? (Among all possibilities)

You can sometimes restructure your sentence to mean the same thing with and without the 「が」 particle.

• 好きな食べ物は、何ですか？
  As for food that (you) like, what is it?
• 食べ物は、何が好きですか？
  As for food, what is it that you like? (Among all possibilities)

However, while the topic particle can only bring up a general topic of conversation, the identifier particle plays a specific role in that it's identifying a particular thing among other possibilities.

• スミスさんは、忙しい。
  As for Smith-san, busy.
• スミスさんが忙しい。
  Smith-san is the one that is busy.

Which teacher do you like the most?

1. どの - which
2. 一番 【いち・ばん】 - number 1; the best; the most
3. かわいい (i-adj) - cute
4. 二年生 【に・ねん・せい】 - second year; sophomore
5. 来年 【らい・ねん】 - next year
6. 楽しみ 【たの・しみ】 - something to look forward to
7. なんで - why
8. 授業 【じゅ・ぎょう】 - class
9. 面白い 【おも・しろ・い】 - interesting
10. 難しい 【むずか・しい】 - difficult

リー： ジョンさんは、どの先生が一番好き？
Lee: As for John-san, which teacher do (you) like the most?

ジョン： 田中先生が好きだね。
John: (I) like Tanaka-sensei.

リー： そうだね。かわいいよね。でも、二年生の先生だよね。
Lee: That's so, isn't it? She is cute. But (she's) second-year teacher.

ジョン： 来年が楽しみだ！
John: Looking forward to next year!

アリス： 私は、山本先生が好きよ。
Alice: As for me, I like Yamamoto-sensei, you know?

ジョン： え？なんで？
John: Huh? Why?

アリス： 授業が面白くない？
Alice: Isn't (his) class interesting?

ジョン： 面白くないよ！難しいよ！
John: It's not interesting! It's difficult!

アリス： そう？
Alice: Is that so?

The one or thing that...

Though it doesn't work all the time, a simple trick to easily distinguish 「が」 is to translate it as "the one or thing that...". This way, it clearly illustrates the 「が」 particle as identifying a particular thing or person.

1. 田中先生が好き。
   Tanaka-sensei is the one that (I) like.
2. 授業が面白くない？
   Isn't (his) class the thing that is interesting?