Basic Principles:

• Main prepositions preceding the nouns describing time are in, on, at, by, until and during;
• In a clause, the prepositional phrase often functions like an adverbial of time (answering the question when), describing when an action (the verb) takes place;
• The prepositional phrase of time can also appear inside a noun phrase, describing when an event (the noun) takes place.

Having explained the nature of prepositional phrases and their positions in a clause in the previous post, it is now easier to explore how they express different meanings. I mentioned that I would talk about time and place; in this post, I will detail those that describe time, more specifically, prepositional phrases starting with in, on and at.

Prepositional phrases telling time function like adverbs of time, modifying the verb by describing when something takes place. The prepositions in, on and at are used based on different levels of specificity: how specific we describe a part of certain extent of time (year [of the decade], season/month [of the year], week [of the month], hour [of the day]). In general, we use in to indicate time of a broader range:

In + year	In 2018, a plane from Indonesia crashed during its flight to Sumatra.
In + month/season/ semester/term	Many people visit Japan to see the cherry blossom in spring/April. We started teaching online in Semester 2.
In + part of the day	In the daytime, cats would usually be sleeping.

On narrows down the range towards a specific day or date:

On + date	Eric’s birthday is on 22nd November.
On + day	My family usually visits the beach on a summer’s day. I have classes on Thursday morning. There will be celebrations on the Christmas day.

At narrows further down to refer to an exact time in which an event takes place. Also, we can use at to indicate festivities:

At + time of day	My marketing writing class usually starts at 1 p.m. on Mondays.
At + festival	Shops will remain open at Christmas.

We can imagine the three prepositions pinning events and actions on different “planes”: in maps them in a larger “area” of time; on zooms in to a particular point of a day; at points to an exact particular point in time. I hope this illustration can be helpful for you to make sense out of these three little word particles.

In the next post, we’ll see these points in time extend into a “line”, and examine how we refer to a period of time with prepositions during, by, and until.