For and by are completely different words and can hardly be compared. I think in Spanish, 'por' is used both as 'for' and 'by' and hence the doubt. e.g. The book was written by him : He is the author of the book the book was written for him : Somebody wrote the book and dedicated it (for example) to him ****************************************************************************************** Otherwise, for : preposition : means 1. with the object or purpose of: e.g. to run for exercise. 2. intended to belong to, or be used in connection with: e.g. equipment for the army; a closet for dishes. 3. suiting the purposes or needs of: e.g. medicine for the aged. 4. in order to obtain, gain, or acquire: e.g. a suit for alimony; to work for wages. 5. (used to express a wish, as of something to be experienced or obtained): e.g. O, for a cold drink! ****************************************************************************************** By has many more meanings. Some are as follows 1. near to or next to: a home by a lake. 2.over the surface of, through the medium of, along, or using as a route: He came by the highway. She arrived by air. 3.on, as a means of conveyance: They arrived by ship. 4.to and beyond the vicinity of; past: He went by the church. 5.within the extent or period of; during: by day; by night.