Every course teaches these three prepositions as three lists: one for clock times, one for days, one for months and years, and then three more lists for places. Six lists, several dozen items, no reason behind any of them. That is a great deal to memorise for something you have to decide in the middle of a sentence, at speed, several times a minute. There is a principle underneath, it covers time and place with the same move, and it removes most of the memorising.
One principle, not six lists
Think of the three as a scale of precision. The first treats what follows as a single point with no size worth mentioning. The second treats it as a surface or a line, something with extent that you sit on rather than inside. The third treats it as a container, something with an inside that surrounds you. Small point, flat surface, enclosing space: that ordering is the whole system, and English applies it to time exactly as it applies it to physical space.
I will see you at six.
I will see you on Monday.
I will see you in August.
The three lines above show the scale running from narrowest to widest. A clock time is treated as a point, so it takes the first preposition. A day is a bounded stretch you can name and sit within, so it takes the second. A month is long enough to be a container you are inside for a while, so it takes the third. Nobody had to decide these one at a time; each follows from how big the speaker treats the period as being.
The same scale for places
Now the same three, used for physical space, in the same order of increasing size.
She is at the door.
The keys are on the table.
The letter is in the drawer.
The first line treats the position as a point, a location with no interior worth mentioning; this is why the same preposition is used for addresses, bus stops and events, all of which are places you are simply present at rather than inside. The second is a surface, and the object rests on top of it. The third is a container with an inside, and the object is enclosed by it. The identical logic that put a clock time first and a month last puts a doorway first and a drawer last.
The scale explains cases that otherwise look arbitrary. A town or a country is large enough to be a container, so it takes the third preposition; a specific address is treated as a point, so it takes the first, even though the address is inside the town. Both are true at once, because the choice describes how the speaker is framing the place, not how large it really is.
Time expressions, from precise to broad
The scale gives you a default for almost every time expression you will need.
at 7.30, at midnight, at lunchtime, at the weekend
on Tuesday, on 14 March, on my birthday, on Christmas Day
in the morning, in April, in summer, in 2019, in the twenty-first century
Everything in the first line above is a point on the clock or a moment treated as one. Everything in the second is a named day or a date, which is a bounded stretch with a name. Everything in the third is a longer period that surrounds the moment you are describing, and note that the last two items show the scale still working at the size of years and centuries. Two items are worth flagging: the weekend appears in the first line here because British usage treats it as a point in the week, and North American usage puts it in the second line instead, which is one of the few places where the two varieties genuinely disagree.
There is also a small closed set of expressions that take no preposition at all, and learners often insert one out of habit. Anything beginning with the words for this, last, next or every stands on its own, and so does the word for the day before today and the day after it.
The difference between in time and on time
These two look like a free choice and are not; they say different things, and the difference matters in work English.
The train arrived on time.
We arrived in time to catch the train.
The first line above means punctual: the event happened at the moment it was scheduled to happen, and the reference point is the timetable. The second means early enough: something was achieved before a deadline passed, with room to spare, and the reference point is whatever had to be done. A train described by the second line would be a train you caught, not a train that ran punctually. The pattern holds for the pair as a whole, so a report handed in punctually uses the first and a report handed in before it was too late uses the second.
Where the principle stops
Being honest about the limits is more useful than pretending the rule is total. Prepositions are the part of English that resists systematising hardest, and there are three places where the scale runs out. Some fixed expressions simply take the preposition they take, and no reasoning recovers it. Some nouns take a preposition determined by the noun rather than by the geometry, which is why they must be learnt as pairs. And transport is a small closed system of its own, where the choice depends on whether you can stand up inside the vehicle rather than on its size.
The practical consequence is that the scale should be your default, not your only strategy. Use it to generate the answer, and treat the exceptions as vocabulary. Where a preposition is fixed by the word in front of it, the reference page listing which small word follows which common adjective is the place to learn those pairs directly, because no principle will produce them.
The mistakes that cost marks
Three errors dominate. The first is using the container preposition with a named day, which happens because a day feels like a period you are inside; the scale puts a named day on the surface instead. The second is inserting a preposition in front of the expressions that take none, which is the most frequent of all and the easiest to fix, since the list is short and closed. The third is transferring the preposition from your own language, and it is worth knowing that this is the single least transferable area of grammar between languages: two languages that agree on almost everything else will disagree here, so a direct translation is more likely to be wrong than right.
The three pieces here that go with this page are the table laying the scale out for time and place together, the lesson on the punctual and the early-enough pair, and the practice set that includes an item where no word belongs at all. Because these three words appear in nearly every sentence you write, a consistent error with them shows up everywhere, and examiners count it under grammatical accuracy rather than style. The same frequency makes them quick to practise: any paragraph you have already written contains several, and checking them takes a minute. Everything else here is arranged on the page that sorts material by level and on the page that sorts it by subject.