I know it's all so random (as this blog is not for Japanese learners), but some people asked me what's the difference between が (ga) and は (wa/ha), so I'm trying to write it down in my point of view as a native Japanese speaker.
So! Let's get started with some example sentences.
わたし は たなか です。
わたし が たなか です。
Both mean 'I am Tanaka.'
Or more exactly, the first one one does. When you use は, you're just introducing yourself. 'What's your name?' 'I am Tanaka.' You're just talking about yourself.
On the other hand, if you use が, you're putting an emphasis on わたし. It's like you heard someone say 'Who is Tanaka?' and so answered 'That's me.' or 'I am the Tanaka you're talking about.'
Not anyone else but I am Tanaka. が has the nuance of 'not anything/anyone else but', and specifies someone/something among others.
Let's have a look at another example here.
わたし は たなか が すきです。
Omfg what the heck is this, は and が in one sentence, this crazy bastard - you might think. Sorry about our confusing language, but anyways I'm gonna explain.
First, let's not think about たなか が and leave it aside. Then the sentence is わたし は すきです, which means 'I like,' and it well makes sense, although we don't know what she likes.
And this は is the same as the は I've introduced above. So here, you're only speaking about your own case. 'I like X, whether you do or not.' 'You might not like X but I do.' 'I like X.'
Then, let's put たなか が back into the sentence. Maybe you already know that this whole sentence means 'I like Tanaka,' but you might be just wondering why it should be が, not を. Actually, を is not wrong, but we just don't usually use it. We use が, because 'I like (no one else but) Tanaka.' You specifically like Tanaka among others.
Then, what if you say 「わたし は たなか は すきです」?
I like Tanaka (but I don't like someone else). I like Tanaka (but not Sato). I like Tanaka (and I'm just talking about him, not about Sato).
It sounds like that. By putting は together with たなか, you only speak about Tanaka, and it implies that it's not necessarily true of others, the case not applied.
In short, は is used when you speak about a particular thing, person, case, etc.「わたし は たなか です」means 'I am Tanaka,' and it doesn't say anything about someone else. You sometimes use は to distinguish something/someone from others, implying 'it's not the case of others.' Look at the sentences below.
わたし は たなか です。 あなたは？
I am Tanaka. And you? --> As for me, I'm named Tanaka. And what's your case?
*Of course we don't really mean it when talking. We only mean 'I am Tanaka and you?'.
▼ These ones have a clear difference.
これ は いちご です。 This is a strawberry. Dunno about others but THIS ONE is.
これ が いちご です。 This is the strawberry. The very strawberry but nothing else.
▼ These may work in both ways. Depends on the context though.
せんせい は いいました。 The teacher said. Whether someone else did or not, he did.
せんせい が いいました。 The teacher said. Not anyone else. It was the teacher who said that.
▼ Could be confusing...?
わたし が すきな ひと は たなか です。
The person I like is Tanaka.
The person I (not someone else) like is Tanaka.
The person who I like is Tanaka. (Not who I hate but who I like. I'm just talking about who I like.)
▼ Don't feel bad even if you don't understand (why)...
わたし は かれし が いません。
I don't have a boyfriend.
I don't have a boyfriend (but my friends do, at least I don't, at least in my case).
I don't have a boyfriend (but I have male friends, I have anything but a boyfriend).
I suck at summarizing stuff so I can't really conclude this whole content, so I'll leave some questions instead, for you to see if you have got the idea of が and は.
(1) 'Where did you get this bag?' 'My dad bought it for me.'
(2) 'Which color do you like?' 'I like this one.'
(3) It was fun last night, but now I'm at work and feeling dead.
「きのう の よる（が・は）たのしかった」
(4) 'Is this your boyfriend?' 'Oh, that one is not. This one is my boyfriend.'
「それ（が・は）ちがうよ。これ（が・は）わたし の かれし」
☟Now, let's check the answers.
(1)「おとうさん が かってくれた」It is my dad who bought the bag (but not anyone else). If you use は, it sounds like 'My dad bought it (but someone else didn't).'
(2)「これ が すき」Cos you particularly choose 'this color' among others. If you use は, it would be like 'I like this one (but not that one).'
(3)「きのう の よる は たのしかった」Cos last night was fun but it's not true of now. If you use が, it's when someone asks you 'When did you have fun the most?' and you say 'Last night was fun,' cos you specifically choose last night among others.
(4)「それ は ちがうよ。これ が わたし の かれし」は cos it's not that you picked up 'that one' to talk about, you didn't choose 'that one' among others. But when you say 'This one is my boyfriend,' you specify someone, so you use が.
So, the question 'Is this your boyfriend?' is「これ が あなた の かれし？」because the person who's asking has specifically chosen 'this' among others, thinking 'this is her boyfriend, no one else.'
Sorry that my explanations aren't well organised, but I hope it somehow helps you comprehend the idea of が and は! Thanks for reading ;)