You may have been told that in is for long times, on for days and at for points. The kind of time word is a better guide. A rule that looks only at that kind is right on 103 of 109 printed lines. The six lines it gets wrong come in three kinds, and one of the kinds is a step the rule already takes for mornings and does not yet take for nights.
Core ideas
Three ideas. The second one is the one learners miss.
The lab sorts the time words into eight kinds and gives each kind one small word. A day takes on: 32 lines, 32 right. A month, a year or a season takes in: 5, 16 and 7 lines, all right. A festival and a clock time take at: 3 and 4 lines, all right. The rule never asks how long the time is.
In the morning is the plain case. Name the day and the word changes: on Saturday morning, on the following morning. The lab finds seven lines like this, and the rule gets all seven. The same idea reaches night, and that is where the rule is still short.
This morning, last night, every morning. Nothing comes before them, so there is nothing to score. The lab lists them under the table and does not count them as misses.
The material
The rule is right on 103 of 109 lines. The other six are not six unrelated cases.
A preposition is a small word that stands in front of a noun and joins it to the rest of the sentence. In, on and at are three of the commonest, and before a time word they are the three that learners most often mix up.
The rule you are usually given is about length: in for long times, on for days, at for points. Look at what it does with Christmas, which lasts days and takes at, and the afternoon, which lasts hours and takes in. Length is not what is deciding.
Sort the time word instead. The lab does, into eight kinds. A day takes on. A month, a year, a season or a morning takes in. A clock time, night, noon, midnight or a festival takes at. That is the whole rule, and you can say it in the time it takes to read it.
Run it on 109 printed lines and it is right on 103, which is 94.5%. The novel gives 77 right of 79. The play gives 9 right of 13, and the modern documents 17 of 17. Six of the eight kinds are right on every line: day, month, year, season, festival and clock time. The two that are not are the ones worth reading.
Morning, afternoon and evening take in, and the table has 22 lines that show it: in the evening, early in the morning. But the table also has seven lines where one of them takes on: on Saturday morning, on Wednesday morning, on the following morning, on the third morning. In each, a day is named or picked out, and the day decides. The rule has a step for this and gets all seven.
Six lines are wrong, and they belong to three kinds. Three put on before night with a day name: on Sunday night, on Thursday night, on Wednesday night. This is the same step again. If a named day beats a morning, it beats a night, and that fixes three lines with no cost, because at is still right on the other seven lines of this kind, six of them at night and one at midnight.
Two lines pick the evening or the morning out by words that come after it: on the evening before my going, on the morning of the day. The lab reads only what comes before the time word, so it cannot see these. A person reads the whole phrase, and the whole phrase names one day. The sixth, at evening parties, is not a time phrase at all.
Below the table the lab lists time words that have nothing before them: the next morning 17 times in the novel, this morning 13, last night 9. They are not scored, because there is no small word to be right or wrong. If you add one, you have made the error.
Two limits. The lab counts spellings and not meanings, so it cannot tell you the right word for a phrase that is not on its list. And 92 of the 109 lines come from texts written in 1813 and 1895. The modern lines are 17 of the 109, and 16 of those are a year. They agree with the rule, but they test only a corner of it.
Interactive
The rule picks the word from the kind of time; the page checks it line by line
| kind of time word | the rule says | lines | right |
|---|
Method
Five questions, in this order.
Then it takes no small word: this morning, last night. Stop here.
Use at: at eight o'clock, at night, at Christmas. One care: when a day is named before night, the day wins, as it does for a morning in the last step, and the phrase is on Sunday night. The lab's rule stops short of this, and the lesson shows the three lines it loses.
Use on: on Tuesday.
Use in: in July, in 2025, in the summer.
Use in, unless a day is named or picked out. Then use on: on Monday morning, on the following morning.
Every phrase below is one of the printed lines, or the same kind of line. Name the kind first, then the small word.
The last phrase is the one that needs care. Morning alone takes in. The following morning names one particular day, and a particular day takes on.
Now the six misses. Three are night with a day name: on Sunday night in the novel, on Thursday night and on Wednesday night in the play. The rule says at for night, and the writers named the day. It is the step that already works for mornings, applied to nights. Two are an evening or a morning picked out by words that come after it: on the evening before my going and on the morning of the day. The rule reads only the words before the time word, so it cannot see them. The last, at evening parties, is not a time phrase: evening describes the parties.
You should be able to state the rule in three parts, read its score off every printed line, name the kinds of line it gets wrong, and tell a time phrase that takes a small word from one that takes none.
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