5 Pairs of Tricky Prepositions in English

Key takeaways

• English prepositions are among the hardest aspects of the language for non-native speakers because their meaning depends on context rather than a single rule.
• The pairs in/on/at follow specific rules for time and place that must be learned separately: in (large containers, periods), on (surfaces, days), at (points, specific times).
• For and since both relate to duration, but for is used with a length of time and since is used with a starting point.
• By and until both relate to deadlines, but by means no later than while until means up to that time.

Most learners reach an intermediate level and still freeze when choosing between correct and incorrect preposition patterns, such as on Monday instead of in Monday, or I have lived here for three years instead of I have lived here since three years.

These mistakes usually come from a real gap in how prepositions work in English compared to other languages. English teachers on italki work through exactly these patterns with learners who have otherwise solid grammar but still hit these specific walls.

This guide covers five pairs of tricky English prepositions: in/on/at for place, in/on/at for time, for/since/ago, by/until, and to/for. Each section includes a contrast table, the core rule, and the most common mistake to avoid.

In / On / At for place

These three prepositions describe location, but each one identifies a different type of space.

In	On	At
Enclosed spaces and areas	Surfaces, floors, streets	Specific points, addresses
In a room	On the table	At the door
In a city	On the floor	At 42 Park Avenue
In a building	On Main Street	At the bus stop
In a country	On the wall	At the top

The core rule:

• In: the location is a container or area that surrounds you (a room, a building, a city, a country)
• On: the location is a surface or linear structure (a table, a street, a floor)
• At: the location is a specific point or address (the bus stop, the door, a precise address)

Most common mistake: I am in the corner when you mean the corner of a room.

When you are inside a space and in a corner (meaning surrounded on two sides), the correct preposition is in the corner. When you are at a street corner (a specific point where two streets meet), the correct preposition is at the corner. Both are correct in their respective contexts.

Working through dozens of location examples in conversation with an English language tutor is the fastest way to build reliable instinct for these distinctions.

In / On / At for time

The same three prepositions apply to time, with parallel logic.

In	On	At
Months, years, seasons, parts of day	Days and dates	Specific times, set moments
In January	On Monday	At 3 pm
In 2023	On 15 March	At noon
In summer	On my birthday	At midnight
In the morning	On Christmas Day	At the weekend (BrE)

The core rule:

• In: a period of time large enough to contain many moments (a month, a year, a season, a part of day)
• On: a specific day or date (a day of the week, a calendar date, a holiday)
• At: a clock time or a fixed moment (a specific hour, midnight, noon)

Most common mistake: on the morning instead of in the morning.

In the morning is correct when referring to the general part of the day. On the morning becomes correct only when referring to a specific morning: on the morning of the interview or on the morning of July 8. The difference is whether you are naming a period (in) or a specific instance of it (on).

Note: American English says on the weekend while British English says at the weekend. Both are correct in their respective varieties.

For / Since / Ago

All three relate to time, but they answer different questions.

For	Since	Ago
A length of time	A starting point	A past point counted backward
For two hours	Since Monday	Two hours ago
For three years	Since 2019	Three years ago
For a long time	Since I arrived	A long time ago
Used with any tense	Only with perfect tenses	Only with simple past

The core rule:

• For: how long something lasts or lasted (a duration). Use it with any tense.
• Since: when something started (a starting point). Use it only with present perfect or past perfect.
• Ago: how far in the past something happened (a distance from now). Use it only with simple past.

Most common mistake: I have lived here since three years.

Since requires a starting point (a moment in time), not a duration. Since three years is incorrect because three years is a duration, not a point. The correct forms:

• I have lived here for three years. (duration with present perfect)
• I have lived here since 2021. (starting point with present perfect)
• I moved here three years ago. (distance from now with simple past)

English verbs and prepositions often need to be learned as fixed patterns, especially when tense and time meaning are involved.

By / Until

Both by and until relate to time limits, but they describe different relationships with that limit.

By	Until
Deadline for completion	End point for continuation
The task must be done before or at that time	The action continues up to that time, then stops
By Friday (finish before Friday)	Until Friday (keep going, stop on Friday)
By midnight (complete before midnight)	Until midnight (work continues, stops at midnight)

The core rule:

• By: a deadline. The action must be completed no later than that point. The action happens before the deadline.
• Until: an end point. The action continues up to that moment, then stops.

The fastest test: Ask yourself whether you are describing completion or continuation.

• Finish the report by Friday. (The report must be completed, no later than Friday.)
• I will be in the office until Friday. (I stay in the office continuously through Friday, then leave.)

Most common mistake: You must submit your application until the 30th.

Submit is a completion verb. You are describing a deadline for a one-time action. The correct form is You must submit your application by the 30th. Until would incorrectly suggest a continuous submission happening up to the 30th.

Contrast: The submission portal will remain open until the 30th. Here until is correct because the portal is open continuously up to the closing date.

To / For

To and for are both used to describe purpose and recipient, but they follow different grammatical patterns.

To	For
Direction and movement toward a destination or person	Purpose, benefit, or intended recipient without movement
Go to school	Buy a gift for her
Send it to me	This is good for your health
Give it to her	I did it for you
I went to the store to buy milk	I went to the store for milk

The core rule:

• To: indicates direction, the destination of movement, or the recipient of a transfer (give, send, show, write)
• For: indicates purpose or the person who benefits (buy, make, cook, prepare)

A useful distinction for purpose: to is followed by an infinitive verb (I went to the store to buy milk), while for is followed by a noun (I went to the store for milk). Both express purpose, but the grammar differs.

Most common mistakes:

• I bought it to you. The correct form is I bought it for you. (You are the beneficiary, not the destination.)
• I did it for help you. The correct form is I did it to help you. (To + infinitive expresses purpose.)
• She cooked dinner to me. The correct form is She cooked dinner for me. (For expresses who benefits.)

For learners who have mastered the basics, these preposition pairs are the type of persistent error that automated tools rarely catch and native speakers notice immediately. Targeted practice with an English tutor online who can catch and correct these in real-time conversation helps close this gap faster than independent study.

Basic English grammar gives you a broader rule base for the sentence patterns that prepositions fit into.

italki has 30,000+ expert tutors trusted by over 10 million learners worldwide. If prepositions are the last gap between your English and full fluency, working with a teacher who drills these patterns in real conversation is far more effective than reviewing rules alone. Book a trial lesson and tell your tutor which pairs give you the most trouble.

FAQ

Why are English prepositions so difficult for non-native speakers?

English prepositions do not map neatly onto the prepositions of other languages. In Spanish, Portuguese, or French, a single preposition often covers territory that English splits between two or three. Learners must learn English prepositions on their own terms rather than translating from their first language.

Is there a general rule for choosing between in, on, and at?

The general rule is scale and specificity. At marks the most specific and smallest scope (a point, an address, a clock time). On marks an intermediate scope (a surface, a day, a date). In marks the broadest scope (an area, a period, a container). When in doubt, think about whether the location or time is a point, a surface, or a surrounding space.

How do I stop confusing by and until?

Ask whether the action is completed (by) or continuing (until). By always signals a deadline for something that happens once. Until always signals the end point of something ongoing. If you can answer how long will this continue? then until is probably correct. If you are setting a deadline for completion, by is correct.