horsto:
I of course rimarked that some transitive verbs are sometimes used in this way, but I'm wondering why.

Simple reason: because the whole concept of "transitivity" is kind of a mess. It is not a category like grammatical gender of some languages that words are arbitrarily put into. It's ultimately an explanation of the meaning of a verb.

horsto:I of course rimarked that some transitive verbs are sometimes used in this way, but I'm wondering why.

There's a section in PMEG that I believe may help explain the sense behind it:

PMEG:Al povas montri tiun, kiu iel profitas aŭ malprofitas de ago. Alivorte al montras tiun, kiu “ricevas” la agon aŭ rezulton de la ago.

Al can show something that in some way profits or is disadvantaged by an action. In other words, al shows something that "receives" the action or result of the action.

There are examples on that page "pardonu al mi", "malhelpis al mi" and others. I would say that informi works in a similar way to pardoni and helpi, both of which are capable of working transitively but can also be used with "al", as in the above examples. I would think of the person who comes after "informi al" as the person "receiving the action" and "profiting from it", as PMEG puts it.

On the subject of dictionaries though, some of them mark verbs as transitive but in doing so they don't necessarily preclude the "al" kind of usage. For example PIV marks "helpi" as transitive but also explicitly states that you can say "helpi al". The same may apply in the case of your dictionary.

tommjames:

PMEG:Al povas montri tiun, kiu iel profitas aŭ malprofitas de ago. Alivorte al montras tiun, kiu “ricevas” la agon aŭ rezulton de la ago.

The main usage of al in this context is certainly in those sentences, where the transitive verb already has an object, f.e.:
Mi donas libron al vi.
.
But for verbs like:
mi dankas vin - mi dankas al vi
mi helpas vin - mi helpas al vi
I think there's no difference in the meaning. Perhaps the different usage originates from languages like the german. As a german you learn that al means Dativ-object and -n means accusative object. And in the german language you use in both case the Dativ.

tommjames:
On the subject of dictionaries though, some of them mark verbs as transitive but in doing so they don't necessarily preclude the "al" kind of usage. For example PIV marks "helpi" as transitive but also explicitly states that you can say "helpi al". The same may apply in the case of your dictionary.

My [url=http://eo.wikipedia.org/wiki/Großes_Wörterbuch_Esperanto-Deutsch]dictonary[/url] doesn't state this for informi, but of course for verbs like danki and helpi.

darkweaqsel:
Simple reason: because the whole concept of "transitivity" is kind of a mess. It is not a category like grammatical gender of some languages that words are arbitrarily put into. It's ultimately an explanation of the meaning of a verb.

I'm sorry, I don't understand. For me transitivity only means that the verb can have a direct object.

horsto:

darkweaqsel:
Simple reason: because the whole concept of "transitivity" is kind of a mess. It is not a category like grammatical gender of some languages that words are arbitrarily put into. It's ultimately an explanation of the meaning of a verb.

I'm sorry, I don't understand. For me transitivity only means that the verb can have a direct object.

The problem is that every verb can have a direct object if its sense permits that. It's not very helpful to mark plaĉi or ridi or iri as intransitive as all of them can theoretically have a direct object (though they aren't used that way normally).

darkweasel:

horsto:
I'm sorry, I don't understand. For me transitivity only means that the verb can have a direct object.

The problem is that every verb can have a direct object if its sense permits that. It's not very helpful to mark plaĉi or ridi or iri as intransitive as all of them can theoretically have a direct object (though they aren't used that way normally).

As you said, they can theorectically have a direct object, but normally they can't. The only exception is this rule, which allows to abbreviate constructions like:

verb + preposition + object
as
verb + object + -n

like for example:
Tio plaĉas al mi - Tio plaĉas min.

If you don't take account of this rule then transitivity makes sense.

horsto:

darkweasel:

horsto:
I'm sorry, I don't understand. For me transitivity only means that the verb can have a direct object.

The problem is that every verb can have a direct object if its sense permits that. It's not very helpful to mark plaĉi or ridi or iri as intransitive as all of them can theoretically have a direct object (though they aren't used that way normally).

As you said, they can theorectically have a direct object, but normally they can't. The only exception is this rule, which allows to abbreviate constructions like:

verb + preposition + object
as
verb + object + -n

like for example:
Tio plaĉas al mi - Tio plaĉas min.

If you don't take account of this rule then transitivity makes sence.

Maybe. Yet the transitivity marker is a bit confusing as I've seen it used as an argument not to use the fully correct plaĉi iun.

horsto:The main usage of al in this context is certainly in those sentences, where the transitive verb already has an object, f.e.: Mi donas libron al vi.

With most of the examples yes, but not all. The ones I quoted (helpi al/pardoni al) neither have nor need one.

horsto:But for verbs like:
mi dankas vin - mi dankas al vi
mi helpas vin - mi helpas al vi
I think there's no difference in the meaning.

Yes, and IMO it's more or less the same with informi vin - informi al vi.

horsto:My dictonary doesn't state this for informi, but of course for verbs like danki and helpi.

In my view it should, and in this case I'm more inclined to trust Reta Vortaro's judgement. The transitive usage is more common, but to me it's just as valid to say "informas al" as it is to say "helpas al". If there's some reason for why it's not then I'd love to know it, because so far it's doing a great job of eluding me.

tommjames:

horsto:My dictonary doesn't state this for informi, but of course for verbs like danki and helpi.

In my view it should, and in this case I'm more inclined to trust Reta Vortaro's judgement. The transitive usage is more common, but to me it's just as valid to say "informas al" as it is to say "helpas al". If there's some reason for why it's not then I'd love to know it, because so far it's doing a great job of eluding me.

Perhaps somebody with a better knowledge of grammar can explain this, it would be interesting also for me.
From my point of view it's easy, because informi in the german language can only get an accusative object, in contrast to danki and helpi.
Perhaps the main meaning of the word informi doesn't emphazise the action of giving something to somebody else, but to emphazise the result, that the subject changes the knowledges (if this is the right word) of the object.

horsto:the subject changes the knowledges (if this is the right word)

In English "knowledge" is a mass noun so you wouldn't use it plurally.. unlike in Esperanto.

tommjames:

horsto:the subject changes the knowledges (if this is the right word)

In English "knowledge" is a mass noun so you wouldn't use it plurally.. unlike in Esperanto.

Haha, does that mean that because you can't have informations you also can't have knowledges?
Sorry for laughing, but that sounds funny to me, I didn't know that (or I forgot it).