は
wa
Put up a sign before you speak: は
The picture Standing a small sign in front of your sentence: this is what we are talking about now. Whatever is not on the sign quietly stays outside.
Where the picture shows up in は
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the sign you put up
私は今日夜勤です。
watashi wa kyō yakin desu
I'm on the night shift today.
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two signs at once
コーヒーはありますが、牛乳はありません。
kōhī wa arimasu ga, gyūnyū wa arimasen
We have coffee, but no milk.
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the one we both know
その書類は課長が持っていきました。
sono shorui wa kachō ga motte ikimashita
The section chief took that document.
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moving the sign
今日は早く帰ってください。
kyō wa hayaku kaette kudasai
Go home early today.
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a sign over the whole thing
日本は夏によく雨が降ります。
nihon wa natsu ni yoku ame ga furimasu
In Japan it rains a lot in summer.
は for what is always so, が for what you notice right now. Looking out the window you say 雨が降っています.