Perdona, ________ antes fue una tontería.

Hola Andrew,

In this sentence, "lo de" is linked to an adverb, "antes", not the rest of the sentence after. We have a similar example in the lesson with "Lo de ayer, ¿es verdad? (What happened yesterday, is it true?)

With "Lo de que..." we need a whole subordinate clause that is linked to that "lo de que".  You will find two conjugated verbs, one for the subordinate and one for the main clause.

"Lo de que me caso con Cristina es broma." (That thing about me marrying Cristina is a joke.)

Here we are having two verbs, one linked to "lo de que" (me caso) and the main verb in the main clause "es". 

However we could say pretty much the same thing with "Lo de" but see how the structure changes:

"Lo de mi boda con Cristina es broma." (That thing about marrying Cristina is a joke.[boda means weeding]). This only needs one verb, "es".

Is this a bit clearer?

Let us know

Inma