What is the difference between qui, que, dont and lequel?

These are the basic relative pronouns which are translated as that or who. Although in English they can be missed out, in French they must be used. e.g. the house that I like- the that here is the relative pronoun, triggering the use of qui,que,dont or lequel. Qui is used to describe a subject, so the word doing the action. e.g. le livre qui est rouge- the book is the one being red therefore making the book the subject of the sentence thus requiring qui. Que is used to describe an object, so the word that the action is being done to. e.g. le livre que j'ai lit- in this case, 'I' am the subject as is the one reading the book therefore making the book the object- the one having the verb done to it. Dont is used in the same way but describes a compliment followed by de. e.g. le livre dont j'ai besoin... avoir besoin is followed by de and so dont must be used instead of que. Finally, lequel is used to replace a compliment followed by any other preposition such as sur, pour, à etc. e.g. c'est le projet sur lequel je travail... this is translated as' the project on which ..' so a compliment -sur- must come before the pronoun thus requiring the use of lequel.