I have a good handle on which verbs take être as an auxiliary, but why does être exist as an auxiliary? Is there some linguistic or historical rationale for existence of two auxiliary verbs in French? For instance, was there some evolution of the role of the past participle from adjective to verb?
Lorsque l'on indique que ce sont les escaliers qui sont gravis ou descendus :
Il a monté/descendu les escaliers quatre à quatre [très vite, en enjambant quatre marches à chaque pas].
Lorsque l'on précise qu'un être vivant utilise les escaliers pour lui-même :
Il est monté rapidement par les escaliers.
Il est descendu de l'escabeau.
Être renvoie à soi, on l'utilise lorsque ce soi agit pour lui-même : je suis tombé, tu es né, il est mort récemment, ... personne d'autre que le sujet en question ne peut réaliser l'action du verbe qui suit.
Source: Our Magnificent Bastard Tongue (2009). [p. 102 Bottom]
Learn a European language, including any Germanic language but Swedish, and note that quite often, while most verbs form their past perfect with the verb have—Ich habe gesprochen (“I have spoken”)—a good little bunch do it with the verb be, too—Ich bin gekommen (“I ‘am come’ ”). Just like in Old English: Learning had fallen away was “Learning was fallen away”: Lār āfeallen wæs.
Marking some verbs with be instead of have is a matter of being explicit about a certain nuance: in the perfect, the verbs marked with be refer, technically, to a state rather than an action; i.e., something that bes. When you say you have arrived, you mean that you have now achieved the state of being there: “I’m here, so let’s get started.” On the other hand, when you talk about how you raked leaves this afternoon, you usually are getting across that you per-
formed the action of raking leaves, not that you have achieved the state of having raked the leaves and are now ready to have your picture taken.
We English speakers think, “Well, yeah . . .” but hardly feel it necessary to split that hair. The other Germanic languages do split it—and Old English did.
But something strange started happening in Middle English, as usual; now it was the be-perfect that was falling away (like autumn leaves). By Shakespeare, be is used with only a few verbs
(“And didst thou not, when she was gone downstairs, desire me to be no more so familiarity with such poor people?” Henry IV, Part II, II, i, 96)
and today, it lingers on only in a frozen form such as The autumn leaves now are gone. Even there, you may well have thought of gone as an adjective (The leaves are red, The leaves are gone), and in any case you can also say The autumn leaves have gone, which, in this case of the grand old Old English be-perfect, they have, as always in English.
[p. 148 Bottom] [The content is also mooted on English SE].
And forget our processing that when we have e-mailed something, an action has been performed while when we have left, a state has arisen in which we are gone. When using the perfect, Old English speakers used be instead of have, with a bunch of verbs that referred more to how things ended up than an event happening. Apparently to us today, “states, schmates”—everything is an action.
Or, what part of speech is gone in She is gone? Call it an adjective—and explain why you can't say a gone dog as you can say a brown dog. She is gone is English's wan gesture toward something robust in its Germanic relatives, in which a whole group of verbs take be instead of have in the past, because they describe something that is more how you are than what you did. To be gone is just that, to be gone. Sure, it is also technically to "have" exerted the action of leaving, but we think more read-ily of the result of the leaving, that one is in the state of being gone. Thus just as French has Il est allé, "He is gone," German has Er ist gegangen. All of the other Germanic languages have the equivalent, or almost all (what's up with you, Swedish?). English crudely forces have on every verb, and while Swedish does, too, that's just one coarseness, as if it happened not to learn to put a napkin in its lap but still went about in double- breasted suits and cultivated orchids. English, in comparison, just-the-facts-ma'am across the board, is Cro-Magnon.
I am not a native speaker but I would try to provide some guidelines. They are not complete since they do not answer the question why naître (to be born) and mourir (to die) belong also to this category. In any case I think it might be better to speak for (a few) intransitive verbs rather than verbs of motion. But if we recall the "maison d'être" we may consider even naître and mourir as "verbs of motion".
In an older form of English, the present perfect tense sometimes used the verb to be as auxiliary. For example, an earlier version of the Bible states:
The Lord is come.
which would be of course
The Lord has come.
in modern English.
This use of be occurred when the conjugated verb was a verb of motion (such as go, run, drive, walk, hurry, come, etc.). So one notices the similarities between English and French languages. This is not a coincidence. I guess it comes from Latin but I am not sure. The thing is that English language during the centuries, for reasons of (personal opinion here) simplicity and establishing itself as an international language among even not well-educated people, has developed a lot. On the contrary, French linguists (again personal opinion here) are more conservative to changes and French language has kept over the centuries its status of a more intellectual language compared to English. But the be/have-être/avoir concept is encountered also in modern German. I do not know much about ancient German but since German and English used to be like brother and sister languages in past (cf. du hast with thou hast and er hat with he hath, for instance) it is not really a surprise. German even today uses the verb sein (to be) as the present perfect tense auxiliary verb with verbs of motion.
Ich bin gefahren = I have driven/I drove = J'ai conduis.
Du bist gekommen = You have come / You came = Tu est venu.
Two such common verbs are marschieren (march) and reisen (travel). Note that common idea between the three languages (Old English, Modern German and French) is the so-called Verb of Motion but the verbs that are conjugated with être/be/sein are not necessarily the same. Cf. reisen and voyager or marschieren and marcher for example.
(Not all the verbs of motion are conjugated with être of course.)