The auxiliary verb （助動詞）～（ら）る is added to the mizenkei of verbs, adjectives, pseudo-adjectives, and some auxiliary verbs. ～る is added to yodan, na-hen, and ra-hen verbs, while ～らる is added to all other conjugations. The expected form of sa-hen verbs, せらる, is generally contracted to さる. ～（ら）る conjugates as a shimo-nidan verb. Because る and ～らる conjugate in the same manner, I’ve treated them as a single auxiliary verb.
It has 4 uses:
- 受身 Passive Voice
This use doesn’t see extensive use in classical Japanese, but it does exist. The subject of a passive sentence in ～（ら）る is almost always animate. The same normal/adversative passive meanings of the modern passive apply.
Ex) 猫犬に追はる。（猫は犬に追われる。）(The cat is chased by the dog.)
- 可能 Potential Voice
In the Heian Era, the potential use of ～（ら）る was generally used in negative constructions or in rhetorical questions where the true meaning was negative. In later periods, it came to be used in the affirmative as well. The classical verb 有り・あり can occur in this form with a modern equivalent of いることができない or いられない or other semantic equivalents. The potential use does not occur in the meireikei.
Ex) 門内にあられず。（門内にはいられない。）(I cannot be inside the gate.)
Ex) 馬より降（くだ）りれざりたり。(馬から降りられなかった。）(He could not get off of his horse.)
- 自発 Spontaneous Mood
This use is used to reflect actions which take place without the speakers conscious intention. Though modern Japanese lacks a separate mood to convey this meaning, we can translate it as 自然に・つい。。。てしまう 。This use also does not occur in the meireikei.
Ex) 我が息子思へば泣かる。（私の息子を思うと、つい泣いてしまう。）(Whenever I think of my son, I can’t help but cry.)
- 尊敬 Honorific
～（ら）る can be used alone to designate the honorific register. It does not need an accompanying honorific verb. Any honorific will suffice for translation.
Ex) 主人明朝帰らる。（主人は明朝帰られる。）(The master will return home tomorrow morning.)