Imperfect usage

[ducviloxi]

I was resting in my hotel when suddenly I heard a big sound

Estaba descansando en mi hotel cuando de repente escuché un gran sonido

o

Descansaba en mi hotel cuando de repente escuché un gran sonido

I was resting in my hotel yesterday morning
I rested in my hotel yesterday morning

Descansaba en mi hotel ayer en la mañana

[Rusty]

I was resting in my hotel when suddenly I heard a big sound

Estaba descansando en mi hotel cuando de repente escuché un gran sonido

o

Descansaba en mi hotel cuando de repente escuché un gran sonido
(Either can be used. The first is the imperfect progressive - was currently in the process. The second is the imperfect tense - was resting (sets stage for something else).)

I was resting in my hotel yesterday morning
I rested in my hotel yesterday morning

Descansaba en el hotel ayer por la mañana (was resting - sets stage for something else, or describes an ongoing event in the past)You didn't translate 'I rested'. The action in that sentence has a start and an end in the past, so the imperfect should not be used.

[AngelicaDeAlquezar]

I agree with Rusty's comments, but just for the sake of regionalisms: "en la mañana" is acceptable in Mexican Spanish; daily speech rarely uses "por la mañana" (it's a bit too formal for us).

[JPablo]

In Spain "por la mañana" is the most colloquial way to express it...

(Mañana por la mañana te espero Juana, debajo de tu ventana...)