Valency categories are a feature of classical European grammar so basic they're taken for granted; they regulate the number of noun phrases that can be associated as arguments with a given verb (or other wordclass, but never mind that for now).
- One argument: “It¹ exists”
- Two arguments: “I¹ seek the Holy Grail²”
- Three arguments: “Sam¹ lent me² this hat³”
Excluding commands with omitted subjects (“stop!”), zero‐argument verbs don't occur in English; Esperanto behaves like a Slavic language by expressing “(it)'s raining” as pluvas.
Compared to the European standard model, English has notably relaxed valency rules, allowing many verbs to occur with any number of arguments (“give”: “please give generously; I gave earlier; cows give milk; she gave me this”). The grammars warn that no such “illogical” behaviour is tolerated from Esperanto verbs – any valency change, no matter how obvious from the accompanying noun cases, must also be signalled with the ig/igh suffixes (E1), like this:
- “give birth to” – what mothers do to babies (marked in the dictionary as an inherently transitive verb)
- the causative form – but rather than meaning “cause to give birth” (a midwife's job), this is used to mean “beget”; that is, what the father did nine months earlier (solo?)
- lexicons translate this not as “give birth” (plain intransitive) but as “be born” – what babies do.
What was that about logic? Meanwhile, reflexives such as “they saw themselves”, which you'd think “logically” would get some similar valency‐modifying suffix, are handled instead as normal transitives with a special pronoun.
The subtleties of valency categories wouldn't matter if they weren't critical to passivisation, which converts any two‐argument verb to a special one‐argument form. Converting an active sentence like “I read the book” (or Esperanto mi legis la libron) involves four steps, in English or Esperanto:
- Simple tenses become compounds: legis → estas leginta
- Active participles become passive: leginta → legita
- The subject is demoted to a “by‐phrase”: mi → de mi
- The object is promoted in its place: libron → libro
So “the book was read by me” = la libro estas legita de mi (note however that de mi can also mean “out of me” or “of me” – i.e. “my book”).
English as usual allows extra possibilities to mislead anglophone Esperantists. Some English verbs mean essentially the same thing whether active or passive (“they burned” vs. “they were burned”; compare “they stabbed” vs. “they were stabbed”). Then there are the three‐argument verbs, which have a choice of promotable objects when passivised (direct or indirect); “Sam lent me this hat” becomes either “this hat was lent to me by Sam” or “I was lent this hat by Sam”. The behaviour of Esperanto indirect objects is similar, but nothing like that latter “passive form” is allowed in Esperanto.
All these complex passivisation rules are so unnecessary, too! Traditional grammarbooks do their best to pretend it's some sort of semantic universal (“When the subject of the verb does not perform the action, it is said to be passive” – drivel! What action are the subjects of such non‐passive verbs as “resemble, enjoy, miss, overhear” performing?) But its only function is to give centre stage to the “Patient” of a situation rather than the “Agent”. Even most modern European languages avoid the passive where possible, and Esperanto shouldn't need the construction at all when it's (potentially) got:
- Topicalising reshuffles – la libron mi legis
- Special vague pronouns – oni/iu(j) legis la libron
- Unspecific subjects – homo(j)/ulo(j) legis la libron
- Zero subjects – (…) legis la libron
- Resultatives – la libro legighis