in/on/at
ɪn / ɒn / ət
A point on the clock, a square on the calendar, a month as a tub
The picture The single point the hands touch on a clock face is at; laying a plan on one square of the calendar is on; putting something inside the tub of a month or a year is in. AT is a point on the clock, ON is a square on the calendar, IN is inside the tub of a month or year.
Where the picture shows up in in/on/at
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a point on the clock
The meeting starts at three.
ðə ˈmiːtɪŋ stɑːts ət θriː
The meeting starts at three.
The hands of the meeting-room clock touching the three exactly
Clock times, which land on a point, take at.
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a square on the calendar
We're closed on Sunday.
wɪə kləʊzd ɒn ˈsʌndeɪ
We're closed on Sunday.
A calendar taped to the shop door with only the Sunday squares crossed through
Days and dates, which have their own square, take on.
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after the span fills
I'll be back in ten minutes.
aɪl biː bæk ɪn ten ˈmɪnɪts
I'll be back in ten minutes.
Turning the hourglass over and stepping out, back when the sand has run through
in ten minutes means after ten minutes, not within them.
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inside a larger span
She was born in March.
ʃiː wəz bɔːn ɪn mɑːtʃ
She was born in March.
A birthday sitting somewhere inside one page of the March calendar, no square named
Months, seasons and years — wide spans — take in.
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pairs that have hardened
I work at night and sleep in the morning.
aɪ wɜːk ət naɪt ənd sliːp ɪn ðə ˈmɔːnɪŋ
I work at night and sleep in the morning.
Working while the window is black, then closing the curtain and lying down as the sun comes in
Learn at night and in the morning whole. Nobody says in night or at morning.
All of it in one picture
Three o'clock is the hands landing on one point, so at; Sunday sits on a calendar square, so on; March is a tub you drop something into, so in. That same in also gives you "in ten minutes" — the picture is a tub of ten minutes filling up before you come back. The last line is a pair to memorise: night hardened into at night, morning into in the morning.