One of the most difficult aspects of learning Japanese is the difference between particles は and が. It is difficult because these two particles do not have counterparts in English. And most of the time substituting one with the other will only change the subtle meaning of a sentence. The sentence will remain grammatically perfect.
For instance, both
A. 私はアメリカ人です
B. 私がアメリカ人です
are grammatically correct. But their meanings are subtly different.

Before explaining the details, let’s review the functions of the two particles:

At least this is what the textbooks say. But because the definitions of Japanese topic and subject are very unclear, e.g. the function of 私は in sentence A is like a subject in English sentence, using topic-subject to explain the difference between は and が will most of the time take you nowhere or even get you more confused.

To make things easier to understand, we will analyze things from another angle and take the situation approach. In other words, we will discuss in which situations we should use は or が.

A. 私はアメリカ人です(Non-exclusive)
B. 私がアメリカ人です(Exclusive)

Sentence A simply tells listeners that “I am an American”. We cannot change it to アメリカ人は私です (American is me), because we know that I am not the only American in the world. In this case, アメリカ人 does not apply to 私 exclusively. It applies to many other people at the same time.

On the other hand, sentence B is only used in a situation where I am the only American, e.g. a group of 8 exchange students from 8 different countries sitting in a room. When the teacher asks, “誰がアメリカ人ですか。(Who is American? )”, you should say “私がアメリカ人です”, because in this situation アメリカ人 applies exclusively to 私. If you use は, you will sound strange as if your teacher ask you about apples and you tell her about oranges. “私がアメリカ人です” in this case can be changed to “アメリカ人は私です”

So when we say トムは犯人だ (犯人means offender of a crime), we imply that there may be other offenders. But if we use トムが犯人だ, then we imply that Tom is the only offender.

C. 鳥は飛ぶ (general)
D. 鳥が飛ぶ (specific)

Sentence C is describing a general fact that birds fly. Listeners will not have a specific bird in their mind. But sentence D is describing a scene that a specific bird or group of birds fly. So when you see a bird fly and want to tell your friend about it, you should use sentence D (actually, progressive form 飛んでいる (flying) sounds more natural). If you use sentence C, you are telling your friend a scientific fact instead of your observation.

E.　トムはいい人です(old)
F.　トムがいい人です(new)

When you use sentence E, you assume that your listeners already know who is トム(Tom). Either you have mentioned it before or both you and the listeners already know him, e.g. Tom is a movie star. If your listeners do not know Tom, then you should use sentence F to introduce him. After this introduction, you can use は, e.g. トムがいい人です。彼は私の同僚です。
For this reason, question words (wh-words in English) cannot be followed by は because they are asking for new information. So after だれ, どこ or いつ, don’t think. Just useが.

G. 山田さんは妻がきれいです (Mr Yamada’s wife is beautiful)

The literal part by part translation of this sentence is: About Mr Yamada, wife is beautiful.
“aはbが…” is a very common Japanese sentence structure. ‘a’ is the topic. ‘b’ is something related to ‘a’. And the ‘…’ part is the information about ‘b’. “aがbは” is extremely rare.

H. 平仮名は読めるけど、漢字は読めない (I can read Hiragana but can’t read Kanji.)

The literal translation is ‘Hiragana, I can read, but, Kanji, I can’t read’. When making contrast between two sentences with opposite meanings, は…は… form is commonly used.

Situation 6: standard sentence structure that がmust be used

The good news is that there are some verbs/ adjectival nouns that must follow が.
They are:

はcan be put behind other particles to form combinations. とは and には in I and J simply mean と and に respectively. は does not change the meaning but indicate the topics of the sentences.

When studying the difference between は and が, we should never think out of context. The choice of は or が depends on situation instead of sentence structure. It doesn’t make any sense at all if we explain the difference between 私は日本人です and 私が日本人です without considering two different situations. And the standard topic vs subject explanation is very difficult for non-native people to understand.

Next time when you ask a native speaker whether your choice of はor が is correct. You must tell him exactly what you want to say, e.g. you want to say a bird in front of you is flying or you want to tell people that birds can fly.