One inescapable phenomenon characterizes the majority of hieroglyphic texts. It may be described in all brevity as the use of past idioms. What comes most readily to mind is the use of a Middle Egyptian idiom in a text composed after the Middle Kingdom, when Middle Egyptian had become a dead stage of the language. For example, Piye´s great stela, dating to the late eighth century B.C.E., exhibits many instances of the auxiliary aHa.n, which was once, a thousand years earlier, part of spoken Egyptian. Its use many centuries later elicits questions of a general nature. How should the use of past idioms be analyzed? Can one write a grammar of the Annals of Tuthmosis III? How should the grammar of the Rosetta Stone be discussed in class and in text editions? The problem is big. It is omnipresent. It is the focus of this paper. The paper has three parts. The first part presents three methodical tenets. The two other parts are two case studies. Bibliography on the use of past idioms in Egyptian texts is listed at the end.