Originally posted by Gambit:

how do you parry?

Parrying an attack to gain a parry bonus is obtained by timing your block just before it hits.
The parry bonus can provide a knockback and stun, as well as making the victim of a successful parry more vulnerable to attack and take more damage for the duration of the stun. This depends also on the parry bonus the blocking weapon/shield applies.
https://valheim.fandom.com/wiki/Parry
https://valheim.fandom.com/wiki/Blocking_(Skill)

Originally posted by Gambit:

also is there a button to split stacks in inventory?

Open inventory and shift left click
https://screenrant.com/split-stacks-resources-inventory-storage-valheim/

Originally posted by Gambit:

and is there a way to select what you want to repair instead of having to repair all?

If repairing tools and weapons, then each has to be repaired individually at the relevant level of workbench.
If repairing buildings and structures, then each part has to be repaired individually but it is easy to see which are in need of repair while you have your build hammer equipped (with the required materials) and in repair mode.
I hope that helps ;)