"It can be used in the singular, for example se tarda, se tardó or in the plural, for example se tardan, se tardaron. Se tardó dos horas en llegar. = Se tardaron dos horas en llegar. It took two hours to arrive."
So both examples mean precisely the same thing? Is there a situation where the meaning would change?
Hola Janet
You can use tardarse in the singular or plural form when you have this type of sentence as long as the time it takes is plural, e.g. two hours, three days...
Se tardó tres meses en construir el puente. (correct)
Se tardaron tres meses en construir el puente. (correct)
Se tardó un día en llegar a un acuerdo. (correct)
Se tardaron un día en llegar a un acuerdo. (incorrect in Peninsular Spanish, because we are talking here about one day.)
I hope this clarifies it.
Saludos
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