ga or wa

Hello, i'm learning japanese, and i need help

I don't understand when we have to use が or は

Exemple: 本 が あります ( hon ga arimasu ) here this is が
本は赤です ( Hon wa aka desu ) and here this is a は

So when we have to use が or は?

It helps to know a bit of English grammar. In the first sentence, book is an indirect object, although I think in English we would count it as a direct object. The subject of the sentence is omitted, as it often is in Japanese. So it really looks like this:

私は本があります

The sentence is about "you". The book is something YOU have. But in the second sentence we are talking about the book.

本は赤いです
私は赤いです

I always urge my students to be wary of omitted subjects. It happens a lot in Japanese, and a lot of confusion between は and が stems for that. It's also worth noting the difference between が and を

自転車が壊れた。
The bike broke.
私は自転車を壊した 。
I broke the bike.

In the first sentence the bike is the indirect object. It broke, but we really don't know how, all we can see is that it is now broken.
In the second sentence, we now know who broke the bike. It is the direct object because it is being directly affected by ME. Instead of focusing so hard on the subject, try looking at the verb to determine which particle to use.

I don't think bringing in English grammar for the "equivalent sentences" in English helps much here :(

Think of the question that was there before the sentences you gave as example.

For example,
- Tsukue no ue ni nani ga arimasu ka? (What is there/what exists on the desk?)
- Hon ga arimasu. (There is a book/A book exists there.)

Here, the existence of "a book/books" is introduced for the first time. "Hon" is the subject of the Japanese sentence. When something new is introduced for the first time, then "ga" as the subject market is used.

- Donna hon desu ka. (What kind of book is it?)
- Hon wa akai desu. (As for the book, it is red.)

When something is the topic (known topic) of the sentence, "wa" is used.

- Dare ga ikimasu ka? (Who is going?)
- Watashi ga ikimasu. (I am going/It's me who is going.)

- Anata wa ikimasu ka? (Are you going?)
- (Watashi wa) ikimasu. (As for me, I am going.)

Ben, I hope you're not an instructor of Japanese...

here is a simple and interesting explanation:
http://www.alc.co.jp/jpn/article/faq/03/19.html

more detailed explanation is here:
http://www.nihongokyoshi.co.jp/manbow/manbow.php?id=916&TAB=1

Now i understand better, i think with time and experience, i will now automatically if is が or は

ty for you help guys :D

I'm just a (fairly new) tutor at my university, Firas. I do have N3 certification, if that helps establish any credibility.

I don't think anything I said was "wrong." I've noticed a lot of people draw confusion from ellipse. And while I used the intransitive, transitive distinction, even though ある is a "be" verb, I believe it still highlights the point that looking at verbs can help one decide which particle to use.

I usually don't tell students about using が as introduction, like AK beautifully explained, because when students are first learning Japanese at my university they do not have to write essays; they are usually answering simple question, and that lack of context is where I think a lot of the confusion lies.

If there is anything I said that was wrong, please let me know! I would hate to have taught students something that was wrong.
An explanation is always more helpful than an insult :p

I meant stative verbs, not linking--or looking as I wrote it, lol.

In all honesty, Ben's way is how I remember how to use wa and ga combined with what AK said. The direct object/indirect object explanation is something I can understand since it was taught to me in my native language. In other words, I can relate it to something I'm familiar with.

Slightly off the topic:
Ben, you seem to have a completely different understanding of English grammar.

In the first sentence, "hon" is the subject of the Japanese sentence.
In English, the "book" would be the direct object (going by your interpretation of "I have a book"). But I disagree - there is no hidden subject. It is only saying "A book exists/is there." In that sense, the "book" is the subject, also in the English sentence.

In the bicycle examples:
In the sentence 自転車がこわれた、the bike is once again the subject of the sentence, in either Japanese or in English. ("The bike broke/gotten broken.")

私は自転車をこわした is "I broke the bike." In English, "I" is the subject, and the "bike" is the direct object. The verb type is different between "kowareru" (something breaks) and "kowasu" (someone breaks something) there, intransitive and transitive verbs, in English grammar.
In Japanese, we refer to "watashi" as the "topic" of the sentence - it is talking about what I did, not who did it. "As for I, (the act I did was) broke the bicycle."

(A native speaker of the Japanese language, qualified as an ESOL teacher and trained as a Japanese language teacher. I have been trained to teach the Japanese language using English to some extent, as my practice students were all English speakers. However because not all learners of the language are English speakers, it is not always recommended to use comparison with English grammar.)

if the word(subject) is replaceable to a pronoun, then use は. otherwise use が.
本があります。
本はあります。
both sentences are correct. the meanings are different.
the first sentence says the answer of "book", 本.
the second sentence says the answer of "exist, there is", あります.

I think that the relations between "が" and "は" are similar to the relations between "the" and "a".

I am the fireman. (私"が"消防士です。)
I am a fireman.(私"は"消防士です。)

When you have to use "は", "the" and "a" don't exist in sentence.

I'm twenty.(私"は"20歳です。)
I'm Japanese.(私"は"日本人です。)

The way I have learned is that Ga　が　 is used as an identifier for something unknown while Wa　は　tells us the subject.

なに　は　がくせい
nani wa gakusei
This roughly translate to "A person named who is a student." or " Is a person named Who a student." if you phrase it as a question.

なに　が　がくせい
nani ga gekusei
This roughly translates to "Who is student?"

If something is unknown, use GA.

Generally, WA is only used once in a sentence, GA is used after WA if we want to use WA anywhere.