Basically, propositions have been taken to be sentences that can be true or false but can never be both. This shows that sentences when they meet some conditions will become a proposition. One major distinction between sentences and propositions is that, a sentence cannot be a bearer of truth value while the proposition does so. This is because propositions have logical connections which enable us to deduce whether it is true or false. Sentences in the kinds of questions, exclamations and the like can not be taken as propositions. Example; What is your name? We can not tell whether this is a true or false sentence.
A sentence(proposition) will convey truth when it expresses a proposition, that is, when a sentence has a reference and it is meaningful. The presence of the reference in a proposition will enable us to connect it with our ideas or beliefs outside our mind in order to ascertain their truth value.
This differ from a sentence which may not have a reference , example, keep quiet! In a proposition like, PHIL 402 students went to Cape Coast last thursday,it is clear what the reference is and with the meaning of the proposition, you can tell whether it is a true assertion or a false one.
Thus, propositions and sentences, they have been used interchangeably, but for the philosophers, they are different terms referring to different things. Simply, a sentence that has a reference and is meaningful expresses a proposition.
Alonzo Church(1956) "Propositions and Sentences" The Problem of Universals, Indiana: University of Notre Dame Press.