が
ga
が is a finger, not a topic
The picture A camera snapping into focus on one thing. Not that one — this one. が points a finger at one thing; は stands up a sign for what you are talking about.
Where the picture shows up in が
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the one picked out
私が行きます。
watashi ga ikimasu
I'll go.
Several hands up in the room, and the light falling on just one of them — mine
は raises a topic; が picks one person out of the others — I'm the one going.
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what just came into view
あ、雨が降ってきました。
a, ame ga futte kimashita
Ah, it's started raining.
The first bead of water hitting the windowpane and catching your eye
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the part that hurts
朝から頭が痛いんです。
asa kara atama ga itai n desu
My head has been aching since this morning.
A body drawn in outline with one spot — the head — glowing red
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what the mind turns to
日本語が少し分かります。
nihongo ga sukoshi wakarimasu
I understand a little Japanese.
A roomful of voices, and one thread of Japanese coming through clear
分かる, 好き and 上手 take が, never を.
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what is standing there
この先にコンビニがあります。
kono saki ni konbini ga arimasu
There's a convenience store just ahead.
One lit convenience-store sign surfacing at the dark end of the street
All of it in one picture
が is the one point that snaps into focus. Raise a hand and that hand is what gets picked; see the first drop on the glass and that rain is what gets picked; name where it hurts and one spot on the body lights up. The language you can follow and the sign at the end of the street surface the same way — one thing out of many. So anything marked が arrives with a pointing finger: not that one, this one.