Questions on how skill leveling works

I am very confused on how some parts of the leveling idea works.

I understand the difference between Class/Weapon/Armor/Racial/World skills in general.

I get how i have to spend a skill point to activate a skill and how it will increase in ability ( I-IV) as i use it.

I get how i can Morph a skill with another skill point.

What i do not understand is how "Levels" work. I understand you have to reach a certain level to unlock specific skills, but this is where i lose track. Do all skill trees unlock as i gain levels or only those i am using? If only those i use, do passive abilities count? If i have to "use" the ability does that mean i have to fill my very limited action bar with 1 skill from each tree just to level that tree?

Note i realise you HAVE to spend a skill point in order to use a skill. I am questioning when a skill is "unlocked" for levels to allow me to spend the point.

Skill lines require an ability to be in your bar in order to gain XP. Armor skills will raise as long as you have a piece of that skill equipped. That's why people suggest you put at least 1 skill from each class line and 1 from the Soul magic line. 

Now for weapon skills I don't know whether you need to have the weapon equipped or not. So if you have 2-hander equipped but no 2-hander skill will that increase? I dunno. Or what if you have a sword and board skill in your bar but you are using a 2-hander. Will sword and board gain xp even though you don't have them equipped? I dunno.

Levels are a bit tricky too. In order to spend a point on a skill ability you have to have the level of that skill at the appropriate level. At first I didn't think your character level mattered, but I know for the crafting skills I wasn't able to put another point into one until my character level was a certain level. So I don't know if that happens with other skill trees or not.

Thanks, that clears some of the fog. Do you know if passives count as "equipped" for skill lines?

It depends on the skill line.  With most skill lines you have to either have a skill from that tree in your action bar or use the weapon/armor/shield.  Mage/Fighter/World skill lines get overall advancement from completing quests associated with those lines, but the individual abilities within those lines will advance as you use them.

Are you sure about that with crafting, Kalizaar?  As I understand it, crafting xp (alternatively known as "Inspiration" to separate it from standard xp) is completely level-independent.  The only level that matters when you are looking at your crafting skills is your overall level in that particular craft; for example, I dig up some iron ore (no xp for that), then refine it into iron ingots (very small amount of Blacksmithing inspiration, which will put me at level 1 Blacksmithing).  I then create an iron dagger (given enough mats, I could create a dagger at any level up to 15 even though my character is, at this point, only level 1).  The creation gives me some more inspiration, and I gain a little bit more by breaking it down again to try and retrieve some of the mats that went into making it.

After several creation/destruction cycles, my Blacksmithing levels up to 2.  I can then spend a point in, say, Metalworking to allow me to forge Steel weapons; I could also, at BS level 2, spend a point in Keen Eye to make ore deposits within 20m of me glow.  The higher skills within the Blacksmithing tree become available as I level up the crafting skill itself, culminating in being able to put a point into Temper Expertise when my Blacksmithing reaches level 10.  All of which, theoretically, I could still do while my character is level 1 (given the skill points from somewhere other than leveling).

Going back to Hygeliac's original question, you will gain xp in your skill lines as you explore, fight and complete quests - I _think_ (not 100% sure) that you gain a small boost in any skill tree that you have opened up, you definitely gain a boost in any skill (and associated tree) which is on your action bar, and a bigger boost according to how often you have used that skill or a skill from that tree.

Short version: weapon and armor skill lines will open up by using an item from that tree, you can then add skill points to expand your 'mastery' of it and be able to add new skills to your bar which will enable you to level it up faster.  Other skill trees require an initial investment of a skill point into one of the associated skills, after which they work in much the same way.  Crafting xp is earned from creation and destruction of items associated with that craft, and leveling it is independent of your other skills.

If u have a skill on your hot bar it will level. Weapons will level if u use them. Same for armor. Passives are part if skill lines, so as u level a skill line u unlock the ability to purchase passives. It is possible to have a skill leveled hight then your level. When you level up u get a point to spend on health/stamina/majicka and a point to spend on a skill line. Sky shards are everywhere and certain quest give skill points to spend on a skill line aswell.

you can morph your skill once it reaches the morphing level and a message will pop up saying suchandsuch can now be morphed and if you open your skills menu you will see the ability to add a skill point just like when you first unlocked the skill only this time it has 2 options to choose branching from the original skill which adds an extra effect to the original skill