Difference Between By and Before (With Table)

In sentence construction, it can be used as a preposition or an adverb.

As a preposition, it serves a variety of purposes:

1.  Indicate a subject or a medium performing a function. In this case, there are three ways in which ‘By’ can be used:
   1. Following a passive verb, e.g. the job was done by him.
   2. Following a noun implying an activity, e.g. The Taj Mahal was the result of years of hard work by the labourers.
   3. Recognizing the author of a book or text, e.g. a novel by Jane Austin. 
2. Identifying the means to achieving an end. For example:
   1. We travelled there by bus.
   2. Tea is made by adding tea and sugar in boiled water.
3. Implying a quantity or a measurement or certain parameters. For example:
   1. She was shorter than her brother by a few centimeters.
   2. She divided it by 9.
4. Implying a period that is in advance or at the last moment of a given deadline. For example:
   1. She must submit her article by Wednesday.
   2. By the time he reached the station, the train was gone.
5. In reference to the physical location of a person or an object that is adjacent to or beside another object or person. For example:
   1. The dead body was found by her building.
   2. Alluvial soil is found by the river Ganga.
6. Related to or according to something or someone. Example:
   1. Anything my father says is all right by me.
   2. He has done his job by her.

As an adverb, it is used to refer to go past or beyond something, e.g. she jogged by her teacher’s house.

‘By’ is also used as an alternative for the noun ‘Bye’.

In a sentence, ‘Before’ can be used as an adverb or a preposition or a conjunction. The term has been derived from the Germanic term ‘bevor’. As a temporal word, it can be used in three ways:

1. A period of time that is in advance or prior to a particular event or time. For example:
   1. She has to complete the syllabus before the commencement of her exams.
   2. The train departed before she reached the station.
2. Until or till the transpiring of a particular event, e.g. it was almost an hour before his father arrived.
3. Implying an event of the past, e.g. though I have never seen her before, I still talked to her.

In the ibid cases, ‘Before’ may be used as a preposition, a conjunction or an adverb.

But as a preposition, ‘Before’ has another usage as well and that is spatial usage. In this case, the term is used to imply the physical location of someone or something that is in front of or ahead of another person or object. For example:

1. She has to give her speech before a large audience.
2. The Shopping Mall is just before the bus stop.

Apart from the above mentioned usages, the term is also used to indicate a preference, e.g.

For a soldier, duty towards the nation comes before everything else.

‘By’ and ‘before’ are some common temporal words that are used in English. But they are often confusions regarding their actual meaning as to what time do they refer to with relation to a particular deadline.

Nonetheless, it is important to remember that ‘By’ should be used in a sentence when the referred task must be completed ‘prior to or at the last moment’ of the given deadline. Conversely, ‘before’ should be used in a sentence when the referred task is supposed to be completed prior to the arrival of the given deadline.

References

1. https://dictionary.cambridge.org/grammar/british-grammar/by
2. https://www.macmillandictionary.com/dictionary/british/by