At level 15 and under, you can reset your skills for free without penalty through the Naturally-Gifted Man, with the exception of your Commoner skills. Above level 15, you must use a Reset Skills to change them.
Sets
Although there exists a slot for a counter-attack set, SoftStar removed these skills from the game.[1] It was originally for the P2P version.[2]
The subbed active skills can be used regardless of the job you are playing, however, a lot of skills require a certain weapon to be equipped. They are further limited by the level of your active job.
You cannot use the ranks of skills that were learned 10, 15 or 20 levels above the level of your current active job, however, you may use lower ranks of the skill.
Certain skills have a reduced effect when used on different jobs as well. Most self-buffs last for a shorter duration when cast on a different job.
Furthermore, regeneration spells have a 10% or 20% increased cooldown when subbed on jobs that aren't compatible with them.
Recommended sub skills
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The recommended skills depend heavily on your active job as lot of skills scale with different attributes. It also heavily depends on what you're trying to achieve. Generally, these are useful sub skills:
Medical Treatment: For additional healing and regeneration, can severely cut down time. Can be used to heal other people as well. Also provides mana regeneration at higher levels. Useful for most jobs, but more useful for people with high Wisdom as it increases the healing from First Aid.
Pet Sounds: Used to heal and buff pets. Very useful if you use a pet as you would otherwise have to use items to restore its health and mana.
Close Combat: Provides mana restoration and an evasion buff, as well as a defense buff and health restoration at higher levels. Useful to reduce downtime on casters.
Sneak Attacks: Provides the ability to increase movement speed and a debuff that shuts down an enemy, as well as the ability to steal from monsters. Useful on solo-ers.
Nature Magic: Provides mana regeneration and a single target freeze, as well as spell reflection and an area freeze at higher levels. Mostly useful in PVP, but can be used for the mana regeneration as well.
The usefulness of most other sub skills are heavily dependant on the active job. You could use Seasonal Magic very efficiently on a Wizard, but it would be awful on a Blademaster.
Skill points
Upon each new level reached, 3 skills points are rewarded. It is also possible to increase your skill points for any job by using springs. Every 10 levels, players are given given a Life Quest that awards a non-tradable spring upon completion. These springs provide an additional 5 skill points when used. It is also possible to purchase tradable springs from the item mall, however they only provide a single skill point as opposed to the 5 from the Life Quest obtained springs. Commoner's Springs are the only springs that can be obtained via other means than completing a Life Quest.
Leveling up skills
Skills can be leveled up when a player has reached a certain level, and has a certian amount of skill points. On the first page of your skills list, these are the first few skills you will be using. When you get a new job, you'll see one or two skills (depending on your job, i.e. Martial Artist). As you level up, you will learn a new skill or more with your skill points, and level up previous ones. On the second page of your skills list, you'll see either more different skills, or better versions of them. They can only be obtained by leveling up a certain skill to level 5. Note that some skills require level 3, 4, or 6 of a certain skill (Such as Mega Medicine, Mock Monsters, Arrow Formation, Snake Formation, Crane Formation, etc.).