'Buy' means to purchase something; 'by' is the preposition for proximity, authorship, or deadline.
Verb. To acquire something in exchange for money. To purchase.
I need to buy a new laptop before the project starts.
She bought three months of advertising space online.
Can you buy some coffee on your way to the office?
Preposition/adverb. Indicates proximity ('next to'), authorship ('by Shakespeare'), means ('by car'), or deadline ('by Friday').
Please submit the form by five o'clock today.
The novel was written by a first-time author.
She sat by the window reading the contract.
Common mistake
Typing error: "We need to by new supplies" (correct: buy). Or "Buy the time she arrived, the meeting had ended" (correct: By the time).
Trick to remember it
'Buy' has a U for 'use' — you use money to buy. 'By' has no U — it is the small preposition with many uses.
Quick rule
Are you acquiring something for money? 'Buy'. Is it a preposition of location, authorship, means, or deadline? 'By'.
Did you know?
The idiom 'buy time' (to deliberately delay something to gain more time) shows how 'buy' can be used metaphorically. 'We need to buy time' has nothing to do with money — time is treated as a commodity you can purchase. This metaphor is thousands of years old in many languages.
Exercise 1 · Pick the right one
All reports must be submitted _____ the end of the business day.
Exercise 2 · Pick the right one
We need to _____ new office chairs before the team doubles in size.
Exercise 3 · Form the word
Are you acquiring something for money? 'Buy'. Is it a preposition of location, authorship, means, or deadline? 'By'.
Typing error: "We need to by new supplies" (correct: buy). Or "Buy the time she arrived, the meeting had ended" (correct: By the time).
'Buy' has a U for 'use' — you use money to buy. 'By' has no U — it is the small preposition with many uses.
Bryan López
Founder of SpeakUP Academy