Maryrrr,

A traditionally fretted dulcimer is not fretted like a guitar.

Once you have tuned the dulcimer to a particular mode, you will always know what mode you are playing in because the dulcimer's diatonic fret system "traps" you in that mode.

With traditional dulcimers the traditional way to change the mode you are playing in is the retune the entire dulcimer.

As long as you don't make chords and as long as you play all the tune notes on the melody string, you will be in that mode.

You can make chords on a dulcimer (I do), but be careful that you don't lose that classic dulcimer modal sound.