en_tn/gen/26/18.md

which they had dug

"which had been dug" (UDB) or "which Abraham's servants had dug."

in the days of Abraham his father

See how this was translated in Genesis 26:15.

The Philistines had stopped them up after Abraham's death

This was the reason that Isaac dug them out. Since this happened first, this sentence can come before the sentence about Isaac digging them out, as in the UDB. Or this sentence can start with "Isaac did this because."

had stopped them up

"had filled them with earth"