Whatever language we speak, before we begin a sentence we have an almost infinite choice of words to use. A, The, They, Whereas, Having, Then, To, Bison, Ignorant, Since, Winnemucca, In, It, As . . . Any word of the immense vocabulary of English may begin an English sentence. As we speak or write the sentence, each word influences the choice of the next -- its syntactical function as noun, verb, adjective, etc., its person and number if a pronoun, its tense and number as a verb, etc. ,etc. And as the sentence goes on, the choices narrow, until the last word may very likely be the only one we can use.