Once you are playing pieces that use a combination of chords and bass runs, it is sometimes difficult to work out which left hand fingers to use for some of the notes.
Where this is not obvious, a composer or arranger will indicate the fingers to be used by placing small numbers near the notes, as shown in the following study for classical guitar. The word allegro means fast, but practice the piece slowly with a metronome until you can consistently play all the notes correctly, and then gradually increase the tempo. Notice the use of slash chords in this piece.