A modifier is a word or short phrase added to the end of a spell to modify the target or effect. The Locomotion Charm, for example, can be modified by including the name of the target. The Wand-Lighting Charm is another example of a spell that can be modified in several ways, though the chosen modifier changes what the spell does, not what the target is. Two such modified spells, the Sunlight Charm and the Dancing Lights Charm, both provide good practice concentrating.

The Sunlight Charm

Incantation: Lumos Solem (‘LOO-mos so-LEM’)
Wand Movement: Single counter-clockwise loop ending with the wand pointed toward the target.
Focus: Target of light
Willpower: Low to very high; determines color and intensity of light
Concentration: High; Target of light

The Sunlight Charm produces a beam of intense light often compared with sunlight and possibly a concentrated form of it. The charm is affected by several factors. The amount of willpower put into the spell determines how powerful the effect is. The spell requires focus to precisely target the beam. It also requires concentration to keep the spell from simply flickering out just after being cast.

Depending upon how much willpower you put into the spell, the beam will range in intensity from a faint white like a flashlight (but visible) to a brilliant light golden yellow at its maximum. Unlike with the unmodified version (IE the Wand-Lighting Charm), this charm does not turn scarlet if over-charged. Care must still be taken as an intense beam can be a danger to your vision, though you should never point your wand toward your own face to begin with.

This charm is also a great way to practice your ability to concentrate. If you cast the spell but are not concentrating upon maintaining the spell to achieve the desired effect, the light will quickly fade out. The spell will only last 5-6 seconds no matter how hard you concentrate, but if you don't concentrate enough the spell won't last for even a second. Managing all of the requirements to cast the spell are what make it a great spell to practice with. 

The Dancing Lights Charm^(u)

Incantation: Lumos Numerosa (‘LOO-mos Noom-air-O-sa’)
Wand Movement: Single counter-clockwise loop, then several rapid counter-clockwise loops.
Focus: None
Willpower: None
Concentration: High. Concentration should be maintained on all the orbs, and determines how many can be created.

An experimental charm that has shown incredible promise, the Dancing Lights Charm produces several spheres of light that can wander directed by the caster's will. Each orb casts about as much light as a candle. The user doesn't have to focus on a particular target, though they need to mentally direct the orbs toward a particular area. Concentration is a key component of the spell. 

The caster must concentrate upon each orb that is conjured. The number of orbs that can be conjured by the spell is limited by the caster's ability to concentrate upon them, though the most skilled caster can only maintain about a dozen at a time. Preliminary testing has determined that a first-year should be able to conjure three to five orbs at a time.

The caster must also concentrate to move the orbs and keep them from simply winking out of existence. Keeping them clustered together is the easiest way to move them all at the same time, though an experienced spellcaster can control them on a more individual level. Typically the caster must choose between more orbs or more control, though practicing concentration increases either of them.