A word doesn’t necessarily belong to a particular part of speech. Depending on how it is used in a sentence, it can belong to more than one. By is no exception: It can function as preposition, adverb, or adjective.
1. By as preposition
That’s the main function of by. Because it’s a preposition, it is followed by a noun phrase or pronoun to form a prepositional phrase.
By can be used as a preposition in following ways:
(Comments that go with examples are in square brackets.)
1. It is used to show the person or thing behind an action. In this use, by is nothing but a passive voice marker:
The city’s electricity supply is managed by a private organization. [The noun phrase a private organization follows the preposition, forming the prepositional phrase by a private organization]
The investigation has been messed up by the police.
The authorities were taken aback by the ferocity of the storm.
2. It is used to show how something is done:
Few passengers from the sunk ship survived by holding on to the floating wreckage.
I paid for rent by cheque.
I’ll travel to Oslo by train.
You can get the discount by filling in the discount code at checkout.
I painted the house all by myself. [all has been added for emphasis. It can be dropped though]
3. It is used to show that something will be done not later than a particular time:
I’ll pay the bill by December 11.
Troops were withdrawn by end of the year.
The actor had given several blockbusters by 30.
4. It is used to show measurement or amount:
In the latest quarter, economy grew by mere 3 percent.
Apple sold the new model of iPhone by the millions in the first few weeks of launch.
The contract labor was paid by the day.
5. It is used to describe someone’s profession, nature, etc.:
He is a banker by profession.
He is quite laidback and easygoing by nature.
6. It is used with day and night to mean ‘during the day’ or ‘during the night’:
When deadlines are on the horizon, I work by the day and by the night.
2. By as adverb
By can provide few types of adverbial information. If you recall, adverbs provide background information such as on time, place, manner, degree, reason, and more. It can be used as an adverb in following ways:
1. It points to a place:
I stopped by for a quick snack on my way to the airport. [It points to an eating joint.]
A family friend dropped by for a quick chat. [It points to a house.]
The mechanic came by in the evening. [It points to a house or a particular address.]
2. It shows movement on a path past something:
The car zoomed by the house.
The parade passed by the dais.
The last year flew by so quickly.
3. It is used to mean that something is being kept aside for future use:
The squirrels are putting nuts by in the burrow for the long winter ahead.
My parents’ generation used to put by money regularly for old age.
4. It shows proximity:
The pharmacy is close by. [Nearby]
The kid wouldn’t be alone; his parents should be close by.
5. It is used in the adverb phrase by and large to mean generally, commenting on the entire sentence:
By and large, the project has progressed smoothly.
By and large, the weather hasn’t been untoward this season.
3. By as adjective
By can sometimes function as an adjective to show that the noun is an incidental or side thing. Note that in this use by is placed immediately before the noun it modifies.
That’s a by issue. [It’s not the main issue. It’s a side issue.]
This is just a by room in the house. [It’s not one of the main rooms in the house.]
If we take the by road, we’ll reach earlier.
Don’t take the by comment seriously.
By has appeared so frequently as an adjective with few nouns that they’re together recognized as a single word. Examples: byelection, bylane, bylaw, byword, bystander, byproduct, bypass, byline, and bycatch.
