keystring If that curved line goes from a note to the same note, then you play those "two same notes" as one continuous note. If the curved line goes from a note to a different note, then you play them legato.
My confusion arises when there is a curved line, connecting intervals or chords, and one note in those intervals or chords is repeated, whereas the others are not.
But I understand now, finally! that - those curved lines are more exact than I thought. If the curved lines connect the same notes in those intervals or chords, those notes should be held, if the curved line are between other notes in the interval or chord, it is a slur and the notes should not be held.