Isn't it strange that only third-person singular present-tense verb inflection remains in English? Is there any reason for this as well as why it changed from -eth to -s?
Third-person singular inflection
When you look at the progression of the English language over time, it doesn't seem so strange. It makes absolute sense.
The process of gradual inflection loss in English has been an ongoing saga for thousands of years.
If we begin at Proto-Germanic, we can illustrate very clearly how we ended up where are are today. Proto-Germanic had conjugated forms for Singular, Plural AND Dual numbers:
Proto-Germanic verbal inflections (present indicative)
Singular
-o
-is
-ith
Dual
-os
-ats
Plural
-am(az)
-ath(az)
-anth
By the time we get to Old English, these had been severely reduced. Most notable: the loss of Dual verbal forms (although the dual pronouns remained using plural verbal forms instead), and the application of the Third Person Plural verbal conjugation for all forms of the plural (OE -ath < PGmc -anth). This suppletive effect is common to all North-Sea Germanic languages, like Frisian and Old Saxon, and is still visible in Plattduutsch (all plural verbs end in -t), and Niedersakisch where they end in -t or -en (-en from Dutch influence).
Old English
-o, -u, -e
-(e)s
-(e)th
-ath
-ath
-ath
When we get to Middle English, they are still basically the same, except that the Plural takes on additional variant forms stemming from the subjuntive (OE -en) and past tense (OE -on), so that we have a ME plural in -eth in the South (< OE -ath) and a -en in the Midlands (< -en; -on) and -es in the North (from either an alteration of -ath oe -es [2nd & 3rd person sing], or from Scandinavian forms). We also see the beginning of third person singular in -es, as above either from Scandinavian or from extension of the 2nd person singular form.
Early Middle English
-e
-(e)st
-(e)th, -(e)s
-eth, -en
-eth, -en
-eth, -en
By Late Middle English (Midland dialect--the forerunner of Early Modern English), the picture looks like this, with loss of terminal -n due to lack of stress
-e
-est
-es, -eth
-e(n)
-e(n)
-e(n)
Then -e is lost, leaving what we basically have today
-
-(e)st ("Thou" form)
-s
-
-
-
Once "thou" is replaced by the form "you" (originally a plural form), it spells wiss doom to the only other remaining verbal inflection in English
-
-
-s
-
-
-
The process of gradual inflection loss in English has been an ongoing saga for thousands of years.
If we begin at Proto-Germanic, we can illustrate very clearly how we ended up where are are today. Proto-Germanic had conjugated forms for Singular, Plural AND Dual numbers:
Proto-Germanic verbal inflections (present indicative)
Singular
-o
-is
-ith
Dual
-os
-ats
Plural
-am(az)
-ath(az)
-anth
By the time we get to Old English, these had been severely reduced. Most notable: the loss of Dual verbal forms (although the dual pronouns remained using plural verbal forms instead), and the application of the Third Person Plural verbal conjugation for all forms of the plural (OE -ath < PGmc -anth). This suppletive effect is common to all North-Sea Germanic languages, like Frisian and Old Saxon, and is still visible in Plattduutsch (all plural verbs end in -t), and Niedersakisch where they end in -t or -en (-en from Dutch influence).
Old English
-o, -u, -e
-(e)s
-(e)th
-ath
-ath
-ath
When we get to Middle English, they are still basically the same, except that the Plural takes on additional variant forms stemming from the subjuntive (OE -en) and past tense (OE -on), so that we have a ME plural in -eth in the South (< OE -ath) and a -en in the Midlands (< -en; -on) and -es in the North (from either an alteration of -ath oe -es [2nd & 3rd person sing], or from Scandinavian forms). We also see the beginning of third person singular in -es, as above either from Scandinavian or from extension of the 2nd person singular form.
Early Middle English
-e
-(e)st
-(e)th, -(e)s
-eth, -en
-eth, -en
-eth, -en
By Late Middle English (Midland dialect--the forerunner of Early Modern English), the picture looks like this, with loss of terminal -n due to lack of stress
-e
-est
-es, -eth
-e(n)
-e(n)
-e(n)
Then -e is lost, leaving what we basically have today
-
-(e)st ("Thou" form)
-s
-
-
-
Once "thou" is replaced by the form "you" (originally a plural form), it spells wiss doom to the only other remaining verbal inflection in English
-
-
-s
-
-
-