Why does my friend’s mortgage say that the borrower shall pay the principal and interest on his loan each month, but I would say that I know he will? What is the difference between shall and will? With, I think, some rightful caution I am going to take us closer to these treacherous grammatical waters, not to explain the depths of the topic, but merely to bring our attention to an interesting distinction we may not suspect English makes.
Let’s set out this way. The distinction between the words shall and will reflects the notion that the future may come about in different ways. The sun, we agree, will rise in the east, and I apparently know the character of my friend so well that I can assure the bank that he will pay his loan on time. Both of these constructions, employing the verb will, mean to assert that the future will unfold in such a way that we will see the sun in the east in the morning, and the bank will have its money when due. This is called the simple future. But take an important note that both these examples have a third-person subject. (English grammar, you remember, has three persons for its verbs: the first to indicate the person speaking; the second, the person spoken to; and the third, the person spoken about. Each person can be either singular or plural. In our examples here, the subjects, of course, are singular.)
To say that the future will unfold in such a way that or it will just happen that means that it is in the very nature of the subject of the verb to act in a certain way: in the world as we know it, the sun has never risen in any direction other than the east, and in the long relationship I have had with my friend, I have always known him to pay his bills and pay them on time. But sometimes, English grammar says, we must compel the future to unfold as we wish it to be—or at least threaten such compulsion. We must, that is, impose an external authority and not leave the future to itself. Very often that authority is the law, and so when the loan document employs the verb shall instead of will with the third-person subject, it means to say that the subject, the borrower, is under obligation to pay his loan on time each month. The bank or the law does not know my friend, the borrower, as I do, and contractual and legal documents are written for everyone unknown. So with the verb shall, English has a way to compel the future when trust is only so great.
Now what makes this a particularly difficult topic is the fact that the verbs shall and will are mixed and matched between the two kinds of future. The simple future has I shall, you will, and he, she, it will; and the determined future has I will, you shall, and he, she, it shall. To say I shall be there tomorrow means, then, that I am putting myself under obligation to conform to the nature of the circumstances, and so it will happen that the future will unfold in such a way that I am there tomorrow. But to say instead that I will be there tomorrow means that I have the determination, the will, to be there tomorrow, however otherwise the future might have unfolded. A tricky topic. And if all that were not enough, there’s the further question of should and would, the past tense forms of shall and will, and even further the questions of how all these words are used in questions and in indirect statements.
Except for the restricted purposes of legal language, modern English for the most part has collapsed this distinction between shall and will, and uses now the verb will for all persons and numbers, whichever kind of future we want to express. But it is a distinction to be aware of, all the more important if we wish to read earlier literature (and some not all that earlier) with a deeper insight and understanding. Proceed slowly with this topic, I would urge, but proceed. We’ve taken a first small step here.