grammar.conjunctions.cause
Because: porque, como, ya que, pues
Porque follows the result (no fui porque llovía); como leads with the cause (como llovía, no fui); ya que / puesto que / dado que are formal 'since'.
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Explication
Spanish has several ways to say 'because/since', and position distinguishes the two everyday ones. Porque ('because') follows the result clause: no fui porque llovía, cerramos temprano porque era feriado. Como ('since/as') leads with the cause, at the start of the sentence: como llovía, no fui.
This positional split is strict: *porque llovía, no fui sounds wrong — front the cause with como, or put porque after the result. The more formal 'since' connectors are ya que, puesto que, dado que: ya que estás aquí, ayudame.
For a same-subject cause you can compress with por + infinitive: lo multaron por estacionar mal ('they fined him for parking badly'). Watch the four spellings: porque ('because'), ¿por qué? ('why?'), el porqué (the noun 'the reason'), and the rare por que ('for which').
Exemples
Since you didn't answer, I came straight over.
Région: global
We closed early because it was a holiday.
Région: global
Since you weren't answering, I texted you; I'm calling because it's urgent.