comparative suffix, from Old English -ra (masc.), -re (fem., neuter), from Proto-Germanic *-izon (cognates: Gothic -iza, Old Saxon -iro, Old Norse -ri, Old High German -iro, German -er), from PIE *-yos-, comparative adjective suffix. Originally also with umlaut change in stem, but this was mostly lost in Old English by historical times and has now vanished (except in better and elder).
For most comparatives of one or two syllables, use of -er seems to be fading as the oral element in our society relies on more before adjectives to express the comparative; thus prettier is more pretty, cooler is more cool [Barnhart].
past-participle suffix of weak verbs, from Old English -ed, -ad, -od (leveled to -ed in Middle English), from Proto-Germanic *-da- (cognates: Old High German -ta, German -t, Old Norse -þa, Gothic -da, -þs), from PIE *-to-, "suffix forming adjectives marking the accomplishment of the notion of the base" [Watkins] (cognates: Sanskrit -tah, Greek -tos, Latin -tus; see -th (1)).
Originally fully pronounced, as still in beloved (which, with blessed, accursed, and a few others retains the full pronunciation through liturgical readings). In Old English already the first and third person singular past tense form of some "weak" verbs was -te, a variant of -de (see -ed), often accompanied by a change in vowel sound (as in modern keep/kept, sleep/slept).
A tendency to shorten final consonants has left English with many past tense forms spelled in -ed but pronounced "-t" (looked, missed, etc.). In some older words both forms exist, with different shades of meaning, as in gilded/gilt, burned/burnt.
word-ending that sometimes distinguish British from American English. In the U.S., the change from -re to -er (to match pronunciation) in words such as fibre, centre, theatre began in late 18c. and became standard there over the next 25 years at the urging of Noah Webster (the 1804 edition of his speller, and especially his 1806 dictionary). The -re spelling, like -our, however, had the authority of Johnson's dictionary behind it and was unmoved in Britain, where it came to be a point of national pride, contra the Yankees.
Despite Webster's efforts, -re was retained in words with -c- or -g- (such as ogre, acre, the latter of which Webster insisted to the end of his days ought to be aker, and it was so printed in editions of the dictionary during his lifetime). The -re spelling generally is more justified by conservative etymology, based on French antecedents. It is met today in the U.S. only in Theatre as an element in the proper names of entertainment showplaces, where it is perhaps felt to inspire a perception of bon ton.