I think all phrases/expressions have a translation or similar phrase in English, but they just use different vocabulary in Spanish.
Poner los cuernos = Put the horns on ==> to cheat on somebody
Se divorcian porque el esposo le puso los cuernos.
Por sis las moscas = For if the flies ==> Just in case
Parece que va a llover. Me llevo la sombrillas por si las moscas.
Ponerse las pilas = Put on the batteries = Get Ready/Be on alert/Put effort more effort on something (well, this one is definitely a bit hard to translate)
Si no te pones las pilas, perderás tu trabajo.
Estrenar = No real english translation,but it means to wear something for the first time.
Estoy estrenando mis zapatos nuevos.
I'm wearing my new shoes for the first time.
Estás estrenando ese vetido?
Are you wearing that dress for the first time?
Desvelado = sleep deprived
And of course, te quiero, which can mean I care for you, but not really i love you. Te quiero pero no te amo. :S
There are some more I'm sure.