Sed tú el que decidas.
"But you be the one to decide."
No quiero ir, sed lo haremos más tarde.
"I don't want to go, but we will do it later."
Ella no llegó a tiempo, sed estaba enferma.
"She didn't arrive on time, but she was ill."
Although sed itself isn't often part of distinct idiomatic expressions, its usage within literary forms might create phrases that can be considered special in literary contexts. However, it does not commonly appear in expressions used in everyday language.
Sed lo que quieras, no cambiaré de opinión.
"But say what you want, I won't change my mind."
Comprender las cosas, sed difícil a veces.
"Understanding things is, but difficult sometimes."
La verdad es dura, sed es necesaria.
"The truth is hard, but necessary."
In summary, sed is a formal and literary conjunction used to express contrast in Spanish. Though not commonly used in everyday speech, it retains historical significance from Latin roots. Its equivalents in modern usage would be "pero" or "mas."