*How do the words 'by' and 'for' differentiate? For is usually used with a direct object... For example: I am going 'for' five hours. In this sentence, for is used to indicate how long one is doing something. By is very different! By is most commonly used as a synonym for 'near': I live 'by' the river. The usage of 'by' in this sentence says where I live in relation to the river... I live 'by' the river. All in all, by is a preposition, which is a word that comes before a noun/pronoun that describes the relation of the subject to something else. For is also more commonly used as a preposition, but can also be used as a conjunction (FANBOYS).