た形
ta-kei
た is not 'yesterday'. It is a stamp already down.
The picture The moment the action finishes, a stamp comes down on the paper. The mark does not come off. た is the box already stamped; る is the box still blank.
Where the picture shows up in た形
-
a mark left behind
日本に来たのは去年です。
nihon ni kita no wa kyonen desu
It was last year that I came to Japan.
A stamp reading 'arrived in Japan' sits in last year's box on the calendar and stays there.
-
the mark of having tried it
富士山に登ったことがあります。
fujisan ni nobotta koto ga arimasu
I have climbed Mt. Fuji.
One 'Mt. Fuji' stamp on the passport page, with no date printed under it.
For experience it is always た + ことがある.
-
a mark just stamped
今、駅に着いたところです。
ima, eki ni tsuita tokoro desu
I've just arrived at the station.
The stamp has just been lifted and the ink is still wet and shining.
-
the stamp you'd better have
薬を飲んだほうがいいですよ。
kusuri o nonda hō ga ii desu yo
You'd better take the medicine.
A finger points at the empty box where the 'took the medicine' stamp has not landed yet.
ほうがいい normally takes た: 飲んだほうがいい, not 飲むほうがいい.
-
the mark you were hunting for
あ、ここにあった。
a, koko ni atta
Ah, here it is.
Your eyes drop onto the thing you were hunting for and the 'found it' stamp comes down with a bang.
You use た even though you found it this second — the searching is what ended.
All of it in one picture
た is not a ruler measuring when something happened; it is the mark that a stamp has landed. In last year's box it is a past event; on a page that records only how many times, it is experience; with the ink still wet, it is something just done. Point at a box the stamp has not reached and it turns into advice — better get that one stamped; watch it land in front of you and it turns into discovery. All five are the same picture, seen from a different distance.