Quick answer
madre mía
Natural: oh my goodness / wow
Literal: “my mother”
How it is used
¡Madre mía, qué frío!Oh my goodness, it’s cold!
Madre mía, qué bien cantas.Wow, you sing so well.
¡Madre mía, el tráfico!Good grief, the traffic!
Madre mía, no me lo esperaba.Wow, I did not see that coming.
Where it is said
It is everywhere in Spain; in Mexico you are more likely to hear <b>híjole</b> and in the River Plate area <b>la puta madre</b>, which is far ruder.
A mini dialogue
— Me han subido el alquiler otra vez.— They raised my rent again.
— ¡Madre mía! ¿Cuánto?— Oh my goodness! By how much?
Watch out
madre mía
It is an exclamation, not an insult and not a way to talk about someone’s mother, so keep the two words together.
Check yourself
When would you say “¡Madre mía!”?