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The particle が marks the subject (shukaku 主格 ). The person/thing that performs an action (with verb predicates) or is what exhibits a certain state (with adjectival/adjectival-noun predicates). You may notice that this definition is an updated version from the one we say in Lessons 9-10. 

The purpose of marking the subject in Japanese is to indicate information that is newly registered to the speaker, and that information is thus being distilled to the listener(s) as new information. This distinction helps が serve as an objective means of making neutral statements and providing answers to questions, as well as asking direct questions such as “what is…?” or “who is…?”

New information

Whereas the purpose of wa は is to topicalize something and bring attention to the comment that follows, the particle ga が is used mostly to present new information in the form of neutral statements. This is especially true with statements regarding the existence of something, the five senses, and simple intransitive verbs (verbs that don't take objects).

i. Existential Sentences

Existential sentences are those that state something exists. Typically, these sentences include information such as location. In English, the subject of an existential sentence is “there” and the item that exists ends up being treated as an object. 

vii. There is a dog in the yard.
viii. There are oranges on the table.
ix. There isn’t a dragon here.
x. There aren’t any pens in the room.

In Japanese existential sentences, the thing that exists is treated as the subject. Furthermore, the “to be” verb for showing existence is carried out by two verbs. Aru ある is used to express existence of (non-living) inanimate objects whereas iru いる is used to express living animate objects.

1. 飴がある。
Ame ga aru.
There is candy.

2. 鉛筆がある。
Empitsu ga aru.
There is/are pencil(s).

3. 鳥がいる。
Tori ga iru.
There is/are (a) bird(s).

4. 牛がいる。
Ushi ga iru.
There is/are (a) cow(s).

5. 魚が｛ある・いる｝。
Sakana ga [aru/iru].
There is/are (a) fish.

Sentence Note: When the verb aru ある is used, “fish” is being treated as a food item that is no longer living. When the verb iru いる is used, the fish is still alive and well.

The subject’s location is marked with the particle ni に. In English, this role may be expressed with “in,” “on,” or no preposition at all. In Japanese, the subject doesn’t have to be the first thing stated. In fact, because anything topicalized with は always takes precedence, it’s not even true that the subject is usually stated first. In this same token, location phrases usually take precedence in existential sentences.

6. あそこに学校がある。
Asoko ni gakkō ga aru.
There is a school over there.

7. 部屋に猫がいる。
Heya ni neko ga iru.
There is/are (a) cat(s) in the room.

8. 机の上に本がある。
Tsukue no ue ni hon ga aru.
There is/are a book(s) on top of the table.

9. テーブルの下にネズミがいる。
Tēburu no shita ni nezumi ga iru.
There is/are (a) mouse/mice underneath the table.

10. 橋の隣に滝がある。
Hashi no tonari ni taki ga aru.
There is a waterfall next to the bridge.

ii. Neutral Statements

Neutral statements are those that describe temporary states and/or actions. They form the objective truth of the recent past, the now, or the near future. The most cited example of this usage of the particle が, however, happens to be Ex. 11. Monkey business is taken seriously in grammar.

11. 猿が木から落ちた。
Saru ga ki kara ochita.
A monkey fell from tree.
Alternatively: It is the monkey that fell from the tree (See Usage 2).

Particle Note: The particle kara から means "from" in this sentence. See Lesson 46.

12. 品質がいい。
Hinshitsu ga ii.
The quality is good.

13. 日食が起きます。
Nisshoku ga okimasu.
There will be a solar eclipse.

14. （彼は）礼儀が悪い。
(Kare wa) reigi ga warui.
His manners are bad.
Literally: As for him, (his) manners are bad.

15. （あなたは）頭がいい。
(Anata wa) atama ga ii.
You’re smart.
Literally: As for you, your mind is good.

2. Exhaustive-listing: It is X that…

There are times when が isn’t meant as a mere statement of new information. Instead, it can also explicitly state that it is “X” that is the subject of the predicate. The “X” can be one entity or several entities, which is where “exhaustive-listing" comes into play. When the predicate describes a static state, one that is not a temporary reality, this interpretation is typically meant. A static state can be expressed with a copular sentence, adjectives, adjectival nouns, or verbs which describe states. In fact, this interpretation reigns supreme over the existential sentences studied above. With が, the things mentioned to exist in a certain place are what’s there.

27. 彼が学生です。
Kare ga gakusei desu.
He is the student.

28. この教科書が便利です。
Kono kyōkasho ga benri desu.
This (is the) textbook that is useful.

29. 波が高い！
Nami ga takai!
These waves are high!

30. このサンマのほうが歯応えが弱い。
Kono samma no hō ga hagotae ga yowai. 
The consistency of this Pacific saury is weak.

Grammar Note: The use of no hō のほう (side of a comparison) intensifies the exhaustive nature of が. Whenever there are two が phrases next to each other like this, the first が phrase is always treated as the subject of the main clause. The secondary が phrase is embedded in the predicate. 

ii. Asking Questions

Exhaustive-listing is a feature of が that is not normally brought out without cause. Meaning, just as is the case for the English equivalents seen in translation, such phrasing is usually brought about some sort of question being asked, for which a direct and substantive answer is required. Unsurprisingly, が is involved in the making and answering of those questions. To ask the direct questions, you add が to an interrogative (question word). The basic question words in Japanese are as follows:

・Dare 誰
・Nani 何
・Itsu/Nanji いつ・何時
・Doko どこ
・Naze 何故

Meaning Note: Nanji 何時 literally means “what time?”

31. どこが病院ですか。
Doko ga byōin desu ka?
Where is the hospital?

Sentence Note: This sentence is not a simple question about where the hospital is. Imagine a person looking at a line of buildings and wondering which is the hospital, perhaps there's even a hint of frustration or urgency.

32. 何故ここに幽霊が存在するんですか。
Naze koko ni yūrei ga sonzai suru n desu ka?
Why is it that ghosts exist here?

Grammar Note: In polite speech, "why" questions must end in n desu ka? んですか. See Lesson 43. 

33. 何がおかしい！？
Nani ga okashii!?

What (is it that) is so funny!?

34a. 誰が社長ですか？
Dare ga shachō desu ka?

34b. 社長は誰ですか？
Shachō wa dare desu ka?
Who’s the company president? (34a)
Who is the company president? (34b)

Grammar Note: Ex. 34a would be appropriate to say when you are somewhere where there is a group of people, one of which you would like identified as the company president by who you’re asking the question to. Ex. 34b, on the other hand, would be used in a situation where the company president is already at the forefront of conversation and the speaker, you, is simply asking the listener about who that person is. This conversation doesn’t have to be held where the company president happens to be at.

35. 明日は｛いつ・何時｝が都合がいいですか。
Ashita wa [itsu/nanji] ga tsugō ga ii desu ka?
As for tomorrow, when is convenient (for you)?

iii. Answers to Questions

Questions brought about with が are typically answered back with the information sought. が provides an exhaustive answer to the question at hand.

36. 「誰が行く？」「僕が行きます。」
“Dare ga iku?” “Boku ga ikimasu.”
“Who’s the one going?” “I’m the one going.”

37. 「何がいい？」「ラーメンがいいでしょう。」
“Nani ga ii?” “Rāmen ga ii deshō.”
“What would be good.” “Ramen would be good.”

Spontaneous Remarks

Whenever someone spontaneously utters something, it is often in reference to some immediate concern. 

38. この薬が効くよ。

Kono kusuri ga kiku yo.
This medicine will work. 

Sentence Note: Suppose you find out a friend has a cold and you have some cold medicine on you. The moment you hear about your friend's condition, you take out the medicine and say this'll help him. This is one way Ex. 38 could be used.  

39. お客さんが来た。
O-kyaku-san ga kita.
Customer(s) are here. 

Sentence Note: You're the owner of a restaurant. It's nearing lunch hour and at last you hear the first guest(s) entering. Just as you hear this, you utter Ex. 39.  

ii. Sense of Discovery

Another application of this is expressing surprise in discovery. This application translates as “X is what Y is…” 

40. あ、これが雪だ！
A, kore ga yuki da!
Ah, this is what snow is!

41. あ、あの人が噂の山田師匠だ！
A, ano hito ga uwasa no Yamada-shishō da!
Ah, that person is the rumored Master Yamada! 

Object Marker with Stative-Transitive Predicates

Having already learned quite a lot about how ga が functions as a subject-marker, we will study its function as an object-marker in Lesson 21.

Despite having rather straightforward roles, the particle が is still very complex once you figure in the layers of nuancing that can be had with it. In our next lesson, we will be covering its archnemesis, the particle は. These two particles do cause a lot of headache for most learners, but at the same time, their differences run deep, and choosing one over the other will at the very least cause a change in nuance and at worse be a grammar mistake. 

Now, our goal here is not to fully dish out these two particles in one go. There is still plenty of elementary grammar that we have yet to cover. However, we will still be looking at over 100 sentences of these two particles once we've finished going through the next lesson's examples, and the reason for this is to at least provide you enough contexts to get the big picture. Learning how to more or less understand these particles will help you figure out when to use them yourself and will become beneficial to you once it is time to learn more about them.

With that, know that learning about these particles is not over. It will take time to master them, so don't fret if you're confused with every sentence. That is a normal reaction to their complexity. Just because you've finished reading this lesson and/or the next one, that doesn't mean you can't revisit the topic.