Isn't it Preterite followed by Imperfect in Indicative?

Question

I understand the main point of the lesson, how the subjunctive adds conditional inference to the sentences.

However, I'm confused with your first example sentences. Isn't it generally the case that in simultaneous past actions in the indicative the preterite is usually followed by imperfect? For instance.......

Mientras ella reñía a su hija, nosotras mirábamos hacia otro lado

Shouldn't that be.......

Mientras ella riñó a su hija, nosotras mirábamos hacia otro lado

Your examples don't seem to follow that rule. Is it perhaps because 'mientras' occurs at the start of the sentence, but in this one it occurs in the middle......

Los niños jugaron en el parque mientras los adultos nos tomábamos un café y charlábamos.

Saludos

Answer

Hola Graeme
You can have different combinations. For example you can have two actions happening at the same time and you can see/consider the two actions are ongoing ones:
Mientras yo veía la tele (=estaba viendo) mi marido escuchaba música (=estaba escuchando)
Or you could have one of each, seeing one of the actions as ongoing and the other one completed:
 
Ella se maquilló mientras yo me duchaba.
She put on her make-up while I had a shower (while I was having a shower)
It'd be more unusual to use the preterite in the mientras clause because we would normally see the conjunction "mientras" (while) as a conjunction for "ongoing actions" more than sudden/completed actions.
Saludos
Inma

Answer

I'm still confused with this!

This came up in today's B2 quiz....

Q: Él se fue de acampada con sus amigos ________ en casa tranquila.

A: mientras yo estudié (imperfect is not available)

Your previous answer stated: 'It'd be more unusual to use the preterite in the mientras clause because we would normally see the conjunction "mientras" (while) as a conjunction for "ongoing actions" more than sudden/completed actions.'

But are we not seeing ongoing actions here? Do we assume they are both sudden/completed actions?

I'm not sure I've ever seen preterite used twice in this way, especially not with mientras!

And, is 'tranquila' being used as an adverb here?