Prepositions: at, on, in

at a specific time: e.g., at 8 o'clock, at 3:30, at midnight
on a day: e.g., on Monday, on July 14, on the third day, on weekdays/weekends
in a period: e.g., in May, in 2001, in the morning/afternoon/evening [but: at night]



Words that are often confused

I will stay here until/till she calls. [I will go after she calls.]
I will be in the office until/till 4 o'clock, [I will leave the office at 4:00.]
I will be in the office by 4 o'clock, [I will arrive at the office not later than 4:00.]

I've worked in this office for six months, (for + a period of time)
I've worked in this office since May. (since + a point in time, i.e., a day, year, etc.)

I worked for a newspaper during the war / the summer. (This tells you "when.")
I worked for a newspaper for four years / for six months. (This tells you "how long.") [not I worked for a newspaper during four years.]

Note: During a period may mean a part of that period or the whole period, e.g., "during the war" can mean part of the war or the whole war. The context usually makes it clear, but if we want to stress or emphasize that an action occupied the whole period, we often use throughout, e.g., It rained throughout the night, [It didn't stop raining.]

I'm going back to Brazil in ten days, [ten days from now]
We arranged our next meeting for April 7th. [to be on April 7th]



Approximate times: past and future

I've known my dentist for ages [for a long time], but I haven't had a checkup
recently/lately [e.g., in the last few months].
I haven't seen Tom recently/lately, [e.g., in the last few days or weeks] I used to go to an Australian dentist, but that was a long time ago. [e.g., 5-10 years ago]
My sister went to the dentist the other day. [a few days ago]
This temporary tooth will be OK for the time being, [for now / until I get a permanent one]



Periods of time

There are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, 7 days in a week, about 4 weeks in a month, about 52 weeks in a year, 10 years in a decade, 100 years in a century, 1,000 years in a millennium.



Time passing: take and last

My English course lasts ten weeks. [It continues for ten weeks.]
How long does the movie last? [How long is it from the beginning to the end?]
It takes me [I need] half an hour to get to school.
We can walk, but it'll take [we'll need] a long time.