How to Use "は" and "が" Correctly

象は鼻が長い。 Elephants have long noses.

While English is a "subject-predicate" oriented language, Japanese is a "topic-comment" oriented language. In the above sentence 象 is a topic and 鼻が長い is a comment. In the comment part, 鼻 is the subject and 長い is the predicate. The main framework of the sentence is composed of a topic and a comment, whereas the subject and the predicate are elements of the comment. In this particular case, the topic has no grammatical relationship to the predicate.

この自転車は私が直しました。 (Literally) This bicycle, I fixed it.

This time, the topic is この自転車 and the comment is 私が直しました. The comment is composed of the subject 私 and the predicate 直しました. The main framework of the sentence is again composed of a topic and a comment, and the topic 自転車 is the object of the predicate 直しました. 

Please note that は follows the topic and が follows the subject. This is the most basic usage of the two particles. Though it seems to be simple, it is difficult to use them correctly. This is because there are a lot of types of sentences without a topic. I will show you an example:

ほら、小鳥が鳴いているよ。 Listen, birds are singing.

Imagine that you woke up one morning and opened the window of the bedroom. You called your wife and said this. It is composed of a subject and a predicate. It includes no topic. When you talk about events that are just occurring, you do not have to place a topic in your sentence. I will show you some other examples.

ひろしは親切だ。 Hiroshi is kind.

This is a typical topic-comment sentence. It chooses ひろし as the topic and describes what he is like. What happens if you replace は with が to make a subject-predicate sentence?

ひろしが親切だ。 Hiroshi is kind.

This sentence is grammatically correct, but it seems strange. Though the predicate 親切だ describes the characteristics of ひろし, he is not the topic. Here is a particular situation in which this sentence might be said:

Makoto: 手伝ってくれる親切な人、誰かいないかな。 Would someone be kind enough to help me out?
Takashi: ひろしが親切だよ。 Hiroshi is kind.

Makoto was looking for someone who would help him. Takashi came up with a person who was likely to help him, and said ひろしが親切だ. In this case, Hiroshi cannot be the topic. The intention of Takashi is not to explain what Hiroshi is like. He wanted to say who would be likely to help Makoto.

クジラは哺乳類だ。 Whales are mammals.

This sentence chooses a particular creature, クジラ, as the topic and defines what it is.
クジラが哺乳類だ。 Whales are mammals.

This also seems strange. Though the predicate 哺乳類だ defines what whales are, they are not the topic. Here is a conversation in which this sentence might occur:

Hiroko: サメとクジラ、どっちが哺乳類なの。 Which are mammals, sharks or whales?
Takashi: クジラが哺乳類だ。 Whales are mammals.

The intention of Takashi is not to explain what whales are. He just answered Hiroko’s question. At any rate, you should remember that the sentences ひろしが親切だ and クジラが哺乳類だ are not usually spoken. Generally speaking, you should choose は when you state a definition of something.

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You must use は in topic/comment sentences and が in subject/predicate sentences. Predicates are very important in deciding which particle to choose.

間もなく野球の試合が始まる。 A baseball game will start soon.
あそこで私の犬がほえている。 My dog is barking over there.
ここから富士山が見える。 Mt. Fuji is visible from here.
風の音が聞こえる。 One can hear the sound of the wind.
この池で魚が釣れる。 Fish can be caught in this pond.

You cannot replace が with は in above sentences. This restriction comes from the verbs that are used. Generally speaking, you should use が when you talk about events just occurring. 

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救急車が通ると、犬はほえる。 Dogs are animals that howl when an ambulance passes by.
救急車が通ると、犬がほえる。 When an ambulance passes by, dogs howl.

The above sentences are both correct, but their meanings are different. The former describes what dogs do when an ambulance passes by. There are two topics: 救急車が通ると and 犬. On the other hand, the latter describes what happens when an ambulance passes by. 救急車が通ると is the only topic, and 犬がほえる is the objective description about what happens.

救急車が通ると、私は心配になる。 When an ambulance passes by, I become worried.
救急車が通ると、私が心配になる。 (This is not incorrect, but is unusual.)

The latter sentence is weird in most cases, because you cannot describe yourself objectively. However, it might be used to say, I, as opposed to others, am always the one who gets worried in my family when an ambulance passes by. I will show you another example of this category:ごめんなさい。私がこの自転車をこわしました。 I am sorry. I am the person who broke this bicycle.

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今年、フィラデルフィアの春はひどい。 Spring in Philadelphia is terrible this year.

A college student in Philadelphia wrote this sentence on her website. Of course, her intention is to describe the spring in Philadelphia this year, so one of the topics is フィラデルフィアの春. 今年 is another topic, since this word is placed at the beginning. Apparently she wanted to write about this year.

今年、フィラデルフィアの春がひどい。 This year, spring in Philadelphia is terrible.

This is a different sentence, whose lone topic is “this year.” It sounds like there is the implicit assumption that somewhere in the United States suffers a terrible spring every year, and it is Philadelphia this year. Please note that this is a topic-comment sentence without は. 

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ひろしは、野球はうまい。 Hiroshi is good at playing baseball but…

This sentence has two topics. The first topic, Hiroshi, dominates the whole sentence, and the second one, baseball, limits the scope. 野球はうまい sounds like talking “only” about baseball, he is good. It strongly implies that he is not good at something else. The nuance interpreted as “only” in English comes from the particle は. Scope limitation is another nature of topic indication.

ひろしは、野球はうまいがテニスはへただ。 Hiroshi is good at playing baseball but not good at playing tennis.

This sentence has three topics. The first topic, Hiroshi, dominates the whole sentence, and the second and third ones, baseball and tennis, limit the scope. 野球はうまい sounds like talking “only” about baseball, he is good, and テニスは下手だ sounds like talking “only” about tennis, he is not good.

今年、フィラデルフィアの春はひどい。 Spring in Philadelphia is terrible this year.

This sentence has two topics: one is 今年, and the other is フィラデルフィアの春. 今年 must be thought of as a topic because it is placed at the beginning. You don’t have to decide which topic is the main one in this sentence. The important point is that what is terrible is limited to “this year” and “spring in Philadelphia.” You can also add another topic and predicate to this sentence as follows:

今年、フィラデルフィアの春はひどいが、サンノゼの春はそんなにひどくない。 Spring in Philadelphia is terrible this year, but in San Jose, not so bad.

If a sentence elongation makes another reasonable sentence like the above one, the original two-topic sentence is proven to be correct. On the other hand, the following sentence is incorrect.この自転車、私は直しました。 (Incorrect)

This can be shown by adding another topic and predicate as follows:

この自転車、私は直しましたが、あなたは直しませんでした。 (Weird) I fixed this bicycle, but you didn’t.

The claim, “I fixed this bicycle,” already implies that other people didn’t do it, so the above sentence is weird. This weirdness is the reason why the original sentence is incorrect.

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むかしむかしあるところに、おじいさんとおばあさんが住んでいました。ある日、おじいさんは山へ柴刈りに、おばあさんは川へ洗濯に行きました。 Once upon a time in a village, there lived an old man and his old wife. One day, the old man went out to a forest to gather firewood, while his wife went to a river to wash their clothes.

This is the beginning of the most popular Japanese folk tale, 桃太郎. You must use が in the first sentence. “An old man and his old wife” cannot be the topic because they are not yet known to the readers. In other words, fairy tales must begin with a subject/predicate-type sentence. On the other hand, you must use は in the second sentence, which actually describes what they did, because the readers expect that the couple should be the topic. I will show you some more examples:
あそこに鳥がいます。あれはツバメです。 There is a bird over there. The bird is a swallow.
パリに友達がいます。その友達は、やさしいです。 I have a friend in Paris. She (or he) is kind.
最近、子供たちの間で妖怪ウォッチというものがはやっています。 それは、テレビアニメに出てくるアイテムです。 The Yokai Watch is popular among children these days. It is an item which appears in a TV cartoon.
が is used for someone or something that is not yet known, and defines it as the subject of a subject/predicate-type sentence. For the same reason, interrogative words need が because they refer to someone or something which is not yet known, and therefore cannot be a topic.
誰がこの自転車をこわしたのですか。 Who broke this bicycle?
何がそんなにおかしいのですか。 What is so funny?
誰はこの自転車をこわしたのですか。 (Incorrect)
何はそんなにおかしいのですか。 (Incorrect)
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私が来ることを、彼は知っていた。 He knew that I would come.
私は来ることを、彼は知っていた。 (Incorrect) He knew that I would come.

We have a simple rule that は cannot be used in subordinate clauses, but this rule can be loosened if you use という just before こと.
たとえ何があっても私が来るということを、彼は知っていた。 (Correct) He knew that I would come whatever would have happened.
たとえ何があっても私は来るということを、彼は知っていた。 (Correct) He knew that I would come whatever would have happened.

The subordinate clause of the former sentence focuses on “who would come”, and that of the latter focuses on “whether ‘I’ would come or not.”私だけがあなたの味方だということを、忘れないでください。 Please do not forget that only I will be on your side.
私だけはあなたの味方だということを、忘れないでください。 Please do not forget that only I will be on your side.

It is “who is on your side” that matters in the former sentence, so it is emphasized no one else will be on "your" side. On the other hand, the latter sentence means that “I” would not betray "you" even if everyone else were against "you."

Although there are many other difficult cases, I would be happy if my explanations help you use the particles は and が correctly.

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