Many other languages have less overt copulas [modal indicants] - they hide away as morphological components of the verb. For example - in olden times the English copula was less visible - it often formed part of the verb itself - as it does in Latin and the modern romance languages. Here are two examples of Old English - one sentence contains a visible copula and one shows the copula hidden in the body of a verb:

"Hie sculon of Godes ierre beon abrogdene" 

"They shall be drawn from God's wrath."

Hie [extantal imbuant] sculon beon [modal indicant] of Godes ierre abrogdene [modal informant]

"Tha het cyning sona niman thone mete . . . and beran thehearfum.

"Then at once the king ordered the food to be taken . . . and to be carried to the poor. "

"Then at once" [adverbial phrase of temporality] "The king" [extantal imbuant] "to be" [modal indicant (1)] "ordered the food to be taken . . . and"[modal informant (1)] [“carried"+ modal indicant (2)] "to the poor. "[modal informant]

 Here we see that the verb "beran" contains the indicant within its structure. The same thing occurs in many other languages.

Now look at a Swedish sentence:

"Flickan springer snabbt." – “The girl is running quickly.”

Here we see that the modal indicant is the “er” which is tacked on the end of the verb root “spring.”  BTW, it is not unusual for parts of speech to be jiggled around in this way – the Scandinavians place the definite article [the] at the END of a word – so if you look at the word “flickan” [the girl] you will see that the ‘n’ on the end is actually the ‘the’ [definite article] stuck on to the end of  “flicka.”

In Latin the indicant often forms part of the verb:

“Puella celeriter currit.” 

“The girl is running quickly.”

“Puella” [extantal imbuant] “celeriter currit.” [modal informant with modal indicant part of the verb “curit.”]

In French the indicant often forms part of the verb:

“La fille court vite.”

“The girl is running quickly.”

“La fille” [extantal imbuant] “court vite.” [modal informant with modal indicant part of the verb “court.”]

There is a sharp dichotomy between two contesting verbs of existence [I speak as a traditionalist now – not as an AITist]

(1)   Wesan = “to be.”

(2)   Beon   =  “to be.”

A clue to possibilities of which was in fact the original verb of existence might be found if we look at the old English form of “be” in the following sentences.  The morphological evidence for the modern “be” being the same as the old “beon” in indisputable.

“Thonne that water astyred bith.” – When the water is troubled.”

Literally translated: “When the water troubled is.”  “Bith is the 3^rd person singular of “beon.”  Now it is not absolutely obvious from the sentence that the writer wished to refer to the existential state of the water at the moment it was disturbed, but I would bet my bottom dollar that it was the troubled STATE of the water and not the ACTUAL existence of the water to which he was referring. So why did he choose the 3^rd person singular of “beon.” And not the 3^rd person singular of “wesan.” [is] to communicate what he meant? I hold that beon referred more to inanimate entities [like water] and wesan applied more to the ‘existence’ of human beings – or as an AITist – the ‘mode of existence’ of humans and perhaps animals.

Let’s look at some more examples:

The forms of “beon” are used particularly to express the future:

“Ic beo gearu sona.”

“I shall be ready soon.”

And an invariable fact:

“Thu ana bist ealra dema.”

“Thou alone art judge of all.”

Incidentally, the 2^nd person form of “beon” – “bist” was still used in Shropshire when I was evacuated there during the war.

To answer your question about wesan and beon, on the evidence of my studies I would guess that before the two verbs got mixed up and partly pruned the verb wesan had more to do with ‘human’ existence.  Look at the words: “Ic eom” =  “I am.”

                 Thu eart = “You are.”

                  He is”    = “He is.” 

Anyway, I am a member of an Old English list and I will put out a general question on the problem.  Many of the list members are extremely scholarly professors of our language and its history.

This is pure speculation on my part, but regarding conjugative agreement in languages generally – I mean worldwide – I see agreement in tense and number as a [reinforcement] ‘data transfer back-up’ mechanism.  For example there is much more chance that you will receive/hear the correct information aurally if in a crowded room I say:

(1)“Sandra is a good-looking woman. ” Than if I said:

(2) ”Sandra good-looking woman. ” Why? – Because in (1) the indicant clarifies two very important bits of information:

(a) That Sandra is one person.

(b) That Sandra is good-looking NOW [and not “was”.] It follows that if ^ was adopted in English in place of “is/are” it would mean sacrificing the important data-transfer support function of number.