I just spent a few hours trying to really get a hang of は and が, so I’d appreciate if someone could verify for me the accuracy of what I spent some time writing!:
冬には気温が下がります
“In the winter, the weather drops”
This conversation is about the wintertime (specifically during the wintertime, bc it is には, which is “in the winter”). The temperature is the new idea/thing you are bringing into the conversation, and the fact that it goes down during the winter is the statement of new information about temperature, which was just introduced into the conversation.
If you were to shift this conversation about winter into a conversation about temperature in the winter and make this the new topic, 気温 would become the topic. If you were to provide new information about 気温, say that the cold weather causes snow to fall, it would go something like this:
寒い気温は雪が降らせる。
“Cold weather causes snow to fall”
Then the conversation continues:
雪が降るのはドライブウェイを掻けなければいけないからめんどくさいなあ。
“Snow falling is a pain, because I have to shovel my driveway (when it happens)”
So what I’m noticing is
If a conversation continues to evolve and go deeper and deeper into an overall topic, the が’s will continue to turn into は’s. See how I started off with 冬 as the topic, and then that branched off into a conversation about cold weather in the context of the winter, making 気温 turn from a subject of the conversation into a topic. Then the conversation branched off into a topic about 雪 falling in the context of cold weather. The subject of this sentence is 私(が), but it is omitted because it is obvious who I am referring to (myself).
See how each subject (が) keeps turning into a subject (は)?
If you were to jump back to a previous topic point in the overall conversation–say you brought up cold weather again–you would use は, since cold weather has already been introduced into the overall conversation and isn’t new to it. Also, it is a topic.
Also, this won’t necessarily always happen. Conversations do not always go in a straight line like this. In my last sentence about snow falling and me having to shovel my driveway, the subject (me) could become a topic by my friend asking me if I get help with shoveling my driveway, for example.
君は助けを求めるために、友達に電話しているの?
“Do you call your friends to request assistance (in shoveling the snow)?”
Or, alternately, the topic could become shoveling driveways
ドライブウェイを掻くのは背を痛められるから、危ないね。
“Yea (ね), Shoveling driveways is dangerous, because it can hurt/put out your back”
It all depends.
When STARTING a conversation, it highly depends. Is it common sense? Is it relevant to the context of the situation? For example, if I were to ask if someone likes snow, this could use は or が. If I randomly asked a friend out of nowhere if they like snow, I would say 雪が好き? because this is a new conversation I just started. They would then reply 雪は好き (or more commonly just 好き) because 雪 is now the topic of this conversation I just started.
However, if we were walking in the middle of a snowy plain and I asked if they liked snow, I would say 雪は好き? because the snow is something we are currently in the middle of experiencing; it isn’t something new being introduced right now (remember, this is about STARTING a conversation. If I switched an already-started conversation to be about snow, then I would use が, bc this is changing the topic to something we were not talking about before)—————
Apologies if some of the sentences sound a bit odd, my Japanese isnt anywhere near advanced yet. I also had a side question.
This is a much more simple sentence, but for some reason it still stumped me. Why does 電車が混こんでいる。 use が and not は? I saw this sentence somewhere. If two people are on a train together, isnt it super obvious the train is crowded if that is the case? So I don’t see why they would have to introduce this idea. Just as I felt like I sort of got it such a simple sentence stumps me lol.
Edit: I wrote this: “By using が instead of は here, I believe that this emphasizes that this particular train is crowded. The listener is having the crowded nature of the train pointed out to them. If I were to use は, it would sound like a general statement about trains” to myself, but I’m not sure if this is correct. I was sort of guessing a bit.
by azzeeter