How to know, when NOT to use accusative?

Question

Some of the things that I really like about Esperanto are the clear rules without exceptions. But there is something that is not really clear and I think, Zamenhof didn't notice.

Zamenhof states in the german version of the fundamento:

Es gibt nur zwei Fälle: Nominativ und Akkusativ; der letztere entsteht aus dem Nominativ, indem die Endung n hinzugefügt wird.

In the english version:

There are two cases: the nominative and the objective (accusative).

But the use of accusative is different in different languages. There are certain verbs in German that always put their object in the dative, even when there’s no preposition.

• helfen helpi
• glauben kredi
• antworten respondi
• ... and many more

Also in Russian (if a person is the goal of motion) and Polish (to help someone, to believe someone) dative is used instead of accusative.

Some examples of Zamenhof using dative where I believe, native English speakers would use accusative:

... bone do, mi kredas al vi, ke la persono, kiu vin interesas, ... ... Nu! se vi al mi ne kredas, interrompis lin la literaturisto ...

Zamenhof (Marta, 1910)

... Li elprenis sian vazeton kaj ŝmiris la pupon per la ŝmiraĵo, per kiu li helpis al la kompatinda maljuna virino, kiu rompis al si la piedon.

Zamenhof (Fabeloj de Andersen 1, 1909)

...Kial vi ne respondas al mi, kiam mi vin demandas? ...

Zamenhof (Dua libro)

So how to know when to use al instead of accusative -n?

Answer 1

The basic accusative in Esperanto (that without direction), is rather simple.

• There is at most one subject (nominative).
• Every other construct without preposition is an object with accusative.

So

• mi kredas al vi
• sabaton mi legos la libron de vi
• helpi unu la alian
• helpi vin
• helpi al vi

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Directional accusative adds accusative with prepositions.

• ni promenas en la ĝardeno = we walk in/inside the garden
• ŝi promenas en la butikon = she walks into the shop

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Then there is the nominative for a transform connected to a verb.

• li opinias ŝin sincera = he thinks her sincere
• ŝi farbas la pordon ruĝa = she paints the door red
• ili elektas lin estro = they elected him chief (or such)
• la sperto igis lin singarda = the experience made him careful

These rules are rather simple though an additional preposition would have done too (en al like into). Not specifying them with much grammar - as I did -, makes them rather basic.

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Answering the question:

(After a comment, I realized I did not answer the real question. The following is a flawed repair.)

For the dative, which in most European languages also comes without a preposition, Esperanto in general uses the preposition al, but sometimes an accusative if there is no direct object.

The cause is simple: different languages have different notions on the object following a verb. For the mentioned German language, verbs can be followed by either accusative, dative and even genitive (Der Dativ ist dem Genitiv sein Tod)

So when using the accusative for an object, it is indeed intended as a direct object in the language. There is just that difference between helpi lin and help al li. Both are correct, helpi al is fundamental, helpi unu la alian also well established.

In general - but for briefness - one should try to use a preposition instead of an accusative: it conveys more, relational information.

• dum la ferioj mi studis - almost the only right form
• pasintan sabaton mi studis
• sabate mi studis
• dum la pasinta sabato mi studis

In the first sentence the relation (during) seems important, and my language sense would not allow an accusative. The other sentences seem to favor briefness though.

Maybe one should mention prepositions as prefixes to verbs.

• li parolas pri la afero
• li priparolas la aferon
• ŝi alparolis lin
• ŝi parolis al li - different meaning

Answer 2

Joop Eggen's (imho) very good and very detailed answer made me realise, that I thought of that question too much in matters of rules instead of meaning.

Of course the most important thing about using prepositions or accusative is the question: which one transports best what I want to say?!

So I think it's worth emphasizing the essence of Joop's answer here again:

In general ... one should try to use a preposition instead of an accusative: it conveys more, relational information.

Answer 3

My experience is, that choosing the right case or preposition is one of the hardest things in languages. Speakers of different languages have different perceptions of the world and therefore what is an intuitive choice for one, can feel totally strange for another.

A good dictionary like PIV shows, what is called grammatical case government in English, that is what cases and/or prepositions are used with a particular word. For the verb helpi PIV gives:

1. (iun, al iu) Faciligi ies taskon, laboron ks, partoprenante en ĝi: Ĉiu helpas sian proksimulon. Ĉu mi povas helpi al vi?
2. (iun) Havigi al iu rimedon kontraŭ danĝero, mizero ks: Helpu k savu min!
3. (al io, kontraŭ io) Esti utila, efika por aŭ kontraŭ io: Kontraŭ lupo ne helpas kalkulo.

So in the case of the verb helpi the accusative is the preferred choice. I don't think, that there is a general rule of "a preposition conveys more, relational information".