Etymology
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-er (2)

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].
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-a (1)
word-forming element which in English is characteristic of fem. nouns and adjectives of Latin or Greek origin (such as idea, coma, mania, basilica, arena, formula, nebula). From Latin -a (plural -ae) and Greek -a, (plural -ai, Latinized as -ae). The Latin suffix also became Italian -a (plural -e), Spanish -a (plural -as). It is represented in Old English by -u, -e, but even then the suffix was fading and by the time of modern English was totally lost or swallowed into silent final -e-.

It also appears in Romanic words from Latin that have been borrowed into English, such as opera, plaza, armada. It figures in scientific names coined in Modern Latin (amoeba, soda, magnolia, etc.) and is common in geographical names formed according to Latin or Greek models (Asia, Africa, America, Arabia, Florida, etc.)

In English it marks sex only in personal names (Julia, Maria, Alberta) and in a few words from Italian or Spanish where a corresponding male form also is in use (donna, senora).
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