Please could you explain why the word “se” is omitted in the example sentence below as in a previous example you wrote “se tardaran”

Question

Is se tardaron correct as well as the agent in the sentence below is unknown and you had written an identical example sentence using se tardaronTardaron mucho en construír los apartamentos

Silvia Piriz · Accepted Answer

Hola Eileen L.
That's an excellent question that gets at a subtle but important point in Spanish grammar! 
You're right that both 'se tardaron' and 'tardaron' can be used, and the difference lies in the nuance of impersonality or reflexivity.
 
When you use:

'Tardaron mucho en construir los apartamentos.'

This is an impersonal sentence where the agent is unspecified or general. It translates roughly to 'They took a long time to build the apartments' or 'It took them a long time', where 'they' refers to an unknown or general group of people (e.g., the construction workers, the people in charge). The verb 'tardar' here is acting transitively or intransitively without the reflexive 'se'.

'Se tardaron mucho en construir los apartamentos.'

When 'se' is added, it often creates a reflexive or pronominal verb construction. While it can sometimes be used impersonally, particularly in certain regions or contexts, 'se tardaron' more literally means 'they delayed themselves' or 'they took their time (on themselves)'. It can emphasize the action as something the subjects did to or for themselves or it can be a part of the pronominal verb 'tardarse', which means to take a certain amount of time.
 
In the context of the lesson 'Spanish impersonal sentences using the ellos form of the verb', the focus is on how the third person plural ('ellos' form) can be used to express an impersonal subject, much like 'one' or 'they' in English when the specific agent is unknown or irrelevant. 
In such cases, omitting the 'se' (as in 'tardaron') often maintains that direct impersonal sense more clearly, without the added layer of reflexivity that 'se' can bring.
 
Think of it this way:

'Tardaron': Impersonal, general 'they'.

'Se tardaron': Can be pronominal/reflexive ('they themselves took time') or, in some especific contexts, an impersonal construction with 'se'.

I hope this explanation clarifies the difference for you!
Buen finde!
Silvia