When to use ”は” or ”が”？
My teacher explains the particles は and が like this:

は is like a subject marker, which I understand. However, she says it's a bit like saying, "Speaking of this, ..." or "By the way, ..." whereas が is kind of just pointing out the subject directly. I understand that you would say, for example: 花が好きです, and not 花は好きです。But sometimes I mix the two particles up when I'm writing sentences. My teacher's definition confuses me because it makes me feel like the two particles are almost interchangeable, but I know they're not. Could anyone explain more clearly what the difference is, or give some examples of when to use each particle and why?

Question

When to use ”は” or ”が”？
My teacher explains the particles は and が like this:

は is like a subject marker, which I understand. However, she says it's a bit like saying, "Speaking of this, ..." or "By the way, ..." whereas が is kind of just pointing out the subject directly. I understand that you would say, for example: 花が好きです, and not 花は好きです。But sometimes I mix the two particles up when I'm writing sentences. My teacher's definition confuses me because it makes me feel like the two particles are almost interchangeable, but I know they're not. Could anyone explain more clearly what the difference is, or give some examples of when to use each particle and why?

Answer

It's less that they are interchangeable and more that they tend to overlap. は is the topic marker (note, in English topic and subject tend to mean the same thing. Here it means whatever answers the questions "what are you talking about?"). が is the (grammatical) subject marker so it goes on whatever or whoever is doing the action in the sentence.

Obviously, sometimes the topic and the subject of the sentence overlap, and when that happens は wins, and you don't see the が. For example:

私はジョーダンです。"I am Jordan.". If someone came up to me right then and asked "what are you talking about?", I'd answer with "私", but I am also the subject of the sentence, so you can see here it overlaps but you only say は.

Now let's see an example where they don't overlap. Yours actually.

私は、花が好きです。"I like flowers". If you translate it more literally it becomes "Speaking of me, flowers are likable", so you can see the topic is still 私 but the thing doing the action is the flowers, so those get が. When the topic doesn't overlap with the subject it doesn't have a grammatical role. You could take it out of the sentence completely and it would still make sense.

So the last thing is that it is actually very common to drop the topic like I mentioned. In the last example, you don't need to say 私は if it's obvious you're talking about yourself to begin with so 花が好きです is actually the more common version (there is a topic it's just being dropped because it's unnecessary). Part of what makes は and が so difficult is they are all about context. A good rule of thumb is が is always used for things that you haven't spoken to someone about yet, it's used when introducing something new. は can only go on a topic that both people are aware of.

It's impossible to give you a complete understanding of the difference in just this comment section. There are whole books written on the subject...uh...topic.

Answer

Just take a look at this, Shiori-san. This explains the difference quite well.
http://japanese.about.com/library/weekly/aa051301a.htm
but of course we native Japanese doesn't always choose which is which like "ahh, in this case, we have to use...ga."
The tips, I guess, is memorizing chunks in your brain and use the phrase over and over again in my opinion.

If you have any question about the article below, feel free to ask me.

Answer

Thank you everyone!