As a preposition of time, “by” means before a specific time.
“by” shows a time limit for something to happen.
An event must happen before a specific time is reached.
We often use “by” for projects with deadlines.
All job applications must be received by the 15th July.
We often use “by” to say when someone will return:
Mark: Where is Jane?
David: She is in London. She’ll be back by Saturday.
We use “by then” to describe a specific time that we have already mentioned earlier in the sentence or in a previous sentence.
Tomorrow is the last day of school. You must finish all the exercises by then.
The TV show starts at 9pm so please finish your homework by then.
Preposition of time: “until”
As a preposition of time, “until” describes a continuous action that stops at a specific time.
“until” describes the duration of an action before a specific time.
I’m staying in New York until Friday.
I’m really tired. Last night I watched a film until midnight.
Mark lived in Manchester until 2010.
“for” + period of time
“for” defines a period of time in the past, present or future.
“for” describes a duration.
“for” means “From the start of the period to the end of the period.”
We use “for” with all tenses.
He has been living in London for several months (present perfect continuous)
Jane has worked here for 10 years. (present perfect)
We will visit Spain for 2 weeks. (future simple)
Mark lived in London for 6 months. (past simple)
Today, the students are reading for 30 minutes. (present continuous)
He reads for 2 hours every day. (present simple)
I had studied French for 5 years. (past perfect)
The form is always “for” + period of time
Examples of possible periods of time are as follows:
for 10 seconds
for 20 minutes
for half an hour
for 3 hours
for a week
for a fortnight (2 weeks)
for 3 weeks
for 2 months
for 3 years
for a decade (10 years)
for a century (100 years)
“since” + point in time from the past
“since” defines a point in time in the past.
“since” means “From a point in the past until now.”
“since” gives the starting point. It defines when things started.
We usually use “since” with the perfect tenses