Passive 'se' vs Impersonal 3rd person plural

Question

I always equate 'impersonal' with 'passive', as they seem to express the same thing. And, I've always used 'se' to express passive. So using third person is new to me, and I'm confused by this lesson and a question Kwiziq asked me to translate:

'They took ages to build the hospital'

among the choices were:

Han tardado mucho en construir el hospital (correct)

Se han tardado mucho en construir el hospital (incorrect)

Can someone explain why the second one is wrong.

Here are two other similar examples I found on Kwiziq that relate to this:

Tardaron mucho en construír los apartamentos = It took a long time to build the apartments.

Se tardó mucho en construir este hospital = It took (them, whoever built it) a long time to build this hospital.

Accepted Answer

Hola Deborah
In fact both sentences would have been right for "it took a long time to build the hospital":

Tardaron mucho en construír los apartamentos
Se tardó mucho en construir este hospital.

We haven't included the second sentence in that quiz but it would also be considered an impersonal sentence using "se" plus the 3rd person singular of the verb tardar. 
Tardar is an intransitive verb, not transitive, so you can use the verbal structure: "tardar en hacer algo" (to take time in doing something) here.
The first sentence using "tardaron" is a type of impersonal sentence where we use the they form of the verb as if we were saying "they" (whoever they are, we are not interested in who, we assume in this case they would be the builders) and it is an active sentence simply omitting a subject. This is where the "explain this" button will take you to explain that question. 
The second sentence using "se tardó" is also an impersonal sentence (not passive). Youl always need a direct object to make a passive "se" sentence. Here we don't have it. 
In passive "se" sentences you use the verb in the 3rd person singular when the subject is singular and the 3rd person plural when the subject is plural. This is easy to see in typical sentences in recipes, for example:

Primero se echa el aceite en la sartén y luego se fríen las cebollas. 
FIrst you  (an impersonal you) put the oil in the pan and then you fry the onions.

If we use the formal passive, although this sounds a bit odd in Spanish...it'd be: "First the oil is put in the pan and then the onions are fried."
Some impersonal and passive sentences in Spanish are really tricky, sometimes there are different opinions about being considered one type or another when the verb is in singular. There is always a lot going on among grammarians in this topic. 
I hope this clarified it a bit.
Inma

Answer

Hola Graeme,
The "se" passive sentences can take the 3rd person singular or 3rd person plural:
In singular:

El otro día se publicó una noticia desagradable.
The other day an unpleasant news was made public.

In plural:

El otro día se publicaron  unas noticias desagradables.
The other day some unpleasant news was made public.

The verb is in 3rd person singular when the passive subject is singular (una noticia - se publicó); the verb is in 3rd person plural when the passive subject is plural (unas noticias - se publicaron)
We can use the passive se with transitive verbs, e.g. publicar (publicar algo).

Una noticia desagradable fue publicada/se publicó.
Unas noticias desagradables fueron publicadas/se publicaron.

The impersonal se is used with intransitive verbs (no direct objects) and it is always formed with se + 3rd person singular:
Aquí en el sur se vive muy bien.
Vivir is an intransitive verb, we cannot say "vivir algo/cosas". 
We can't turn this into a passive sentence, you cannot say:
Here in the south it is lived very well. (it doesn't make sense) 
So this is not a passive sentence, it is an impersonal sentence using "se".
Other impersonal sentences using se of this type could be:

En este colchón se duerme de maravilla. (One sleeps beautifully on this mattress.)
En mi oficina de antes se trabajaba mejor. (In my previous office one could work better)

Again, you cannot turn these two sentences into a passive one. They are intransitive verbs.
I hope this clarified it.
Inma

Answer

Hola Inma,I'm having similar problems as Graeme, and I have a follow up question to this example:Tardaron mucho en construír los apartamentos = impersonal sentenceSe tardó mucho en construir este hospital = passive sentenceSo as I understood it now, the 3rd person singular se sentence (tardó) was not correct for this specific test, because it was asking to translate the English sentence into Spanish using an impersonal sentence. And since tardar is a transitive verb, it can only form a passive se sentence and not an impersonal one...?But if the task had been to use a passive sentence instead, it would have been correct to use the 3rd person se sentence with tardar?Coming back to the impersonal sentences. What is the difference when using either the 3rd person se sentence vs. the 3rd person plural one? Is it that the first is only working with intransitive verbs and the latter (also?) with transitive verbs?Thanks as always :)