Does an argument consist of sentences or prepositions? There should not be any confusion on this point. The sentences, which figure in arguments, express propositions. Strictly speaking proposition is the constituents of arguments, should, however, be clear about the distinction between sentence and proposition.
Firstly, all sentences do not express propositions. Only declarative or indicative sentences press propositions. Questions, (viz., How old are you? "Who is your father?", 'Are you a student?)', commands ('Go there', 'Get out', 'Take whatever available') and exclamation (viz, what a book!') are sentences but they do not express any proposition. Such sentences do have any truth value as they do not assert or deny anything.
Secondly, a sentence is a linguistic entity belonging to a specific language, whereas propositions are logical entities having no specific allegiance to any particular language. Of course, to express a proposition we always need a sentence but a proposition is different from a sentence.
Two or more sentences belonging to the same or to different languages may express the same proposition. For example, "Ram killed Ravan" and "Ravan was killed by Ram" are two different sentences in English but both express lie same proposition Because, the state of affair described by the first sentence is the same as mat of the second sentence.
So far as the proposition is concerned these two sentences express lay same proposition. Similarly, the sentence "Ram killed Ravan" can be translated into any other language like Oriya, Hindi or Sanskrit, and the corresponding sentences in these languages would express the same proposition.
Thirdly, the same sentence may express different proposition uttered at different times and in different places. For example, the sentence "The present Prime Minister of India is a bachelor" uttered in the year 1994 would express a false proposition whereas the same sentence uttered in the year 2002 would express a true proposition.
In other words, the states of affair expressed by the two utterances of the same sentence at different times are different. Even if the sentence is the same, the propositions expressed by the sentence at difference times would be different. Thus propositions are distinct from sentences.