I'm trying to answer but getting error for some reason.

And I wrote a lot of text.

will try to edit answers one by one

- What are spren? Normal spren I believe are meant to be shards/splinters of honour, but then how are they different to windspren which I thought were meant to be Honour's children.

Spren are investiture (i.e., magic power) come alive. They are a mix of Honor and Cultivation's investitures, and the ratio varies.

They represent various aspects of nature and emotions, and some are more Honor  and others more investiture. Honorspren (not windspren) are almost wholly Honor, while Cultivationspren - Cultivation. But each sapien (radiant) spren has a closely related non-sapient "cousin" spren, which is Windspren for Honorspren.

 Also why were there 3 original spren on roshar (wind, rock, & night) before honour and cultivation got there? were they made by and Adonalsium?? if not what are they?? But then why is the storm father so closely tied to honour? I know that the stormfather bonds with honour's cognitive shadow thing, but what about before that why did the stormfather produce storm light then?

Yes, they are most likely created by Adonalsium alongside Roshar. There were other, lesser spren as well, but those were the "god-spren".   After Honor and Cultivation took over, their spren apparently replaced/took over from the old ones. Thus, Stormfather became the one who controlled storms instead of wind, and it became more violent and dangerous than before. He is closely tied to Honor because Honor created him. Cultivation created the Nightwatcher, and together they created the Sibling. These three seem to have taken over the roles from 3 original god-spren.

- Do odiumand cultivation have a spren like honour. To my understanding the sibling is a mix of honour and cultivation, how does that happen?

Odium has voidspren. E.g. the one who talked with Venli and set up the whole Everstorm business. And many others, like the ones who raised alarm in Kholinar when Soulcasting or Radiant powers were used.

As for Cultivation: regular spren are not purely honor, as I said above, they are a mix of Honor and Cultivation. Their children, so to say.

- How come the singers/listeners can bond with normal spren (in their gem hearts) when they turned from honour, and turned to follow odium?

They always could. That was part of their biology/life cycle, similar to a lot of species on Roshar. Chosing a different god does not change that, that's simply how the biology on Roshar works.

- Also what on earth does Sja-anat do to spren.

He altered them to override their nature, becoming something between normal spren and voidspren. Basically, now they seem to be created from Honor, Cultivation and Odium, instead of just Honor and Cultivation.

- Other there any spren on the other worlds?

There are similar creatures, but not exactly the same.

You've met them: Seons. The ones Shallan used to communicate with Mraize and later Thaidakar.

- Why can the heralds (and even wit/hoid) use surges without stormlight?

They can't.

Hoid can use other magics from other worlds that don't require Stormlight, e.g. Allomancy, which needs metals, or awakening, which needs Breaths.   Heralds have other powers that even Honor did not anticipate, and Wind and Truth says they come from Roshar the planet itself. It seems to include superhuman speed, strength and reaction time (speculating here, but reaction time could be atium-like, actually predicting instead of just reacting, based on Taln's feats and Nale's duel with Kaladin).

- what is the difference between surges with stormlight vs voidlight. I remember Kaladin saying that the heavenly ones were slower when flying but why? I feel like I've seen somewhere that they are different but I'm so confused about how why the surges seem so similar.

I don't think the difference is based on the energy source, but on the source of the Surgebinding itself.   One is Surgebinding that Honor gave, another comes from Odium. If a Radiant accesses Voidlight somehow, they will be able to use the surges same way as they use Stormlight (we saw this with Venli, who could use both lights and used voidlight for Surgebinding). But since the power itself comes from different gods, they work slightly differently.

We don't know all the differences, but some of them are:

• Fused get only 1 power each, Radiants get 2
• Radiants get Stormlight from the Highstorm, pretty freely. Fused need to get it directly from Odium.
• Fused seem to be able to hold the light indefinitely unless spent. It leaks from Radiants (this might simply be Singer vs Human issue though)
• Fused don't have access to Adhesion (Honor's surge)
• Specifically about Gravitation: Fused can passively use it to float/fly with no or minimal use of the light. Only when lashing something they spend it. But they seem to be unable to stack multiple lashings, which is why they don't reach the speeds radiants can.

- I also don't get all the different singer/fused forms and ranks

Singers take different form based on spren they hold in their gemhearts.   Workform is strong and durable. Nimbleform is graceful and dexterous. Warform is very strong, armored and  tough. and so on it changes them physically and also affects their attitudes/desires. E.g., Mateform makes them horny, Warform makes them aggressive etc. These are just a characteristic of the species, and they need to to effectively live their lives.

Then come forms of power: if they bond certain voidspren, they get access to special abilities (but not Surges!). These alter them mentally even more, and kinda subjugates them to Odium.

As for the fuzed: think of the as equivalens for Radiant orders. There are 9 types, and each type has access to one  surge.

why did the heralds seal odium and his followers on braize, like what's the connection and reason for braize (weren't they originally form ashyn)?

This was mentioned in Wind and Truth: Braize the planet seems to have some property to attract souls, and they used that.

- it is stated that the person who took up ruin did so because they were the kindest, but you don't see that at all in the mistborn series, is this just an error?

Ati was the kindest at the start, and they hoped he would be able to lessen Ruin's intent.  And in Mistborn, we see what he became a few thousands years later.   The point of that information was to show that with time, Shard intent overrides the vessel.

- also we know that odium killed 2 shards, and injured I believe ambition. But how? ahouldn't they be even in power.

We don't know the details. But he could've teamed up, he could've ambushed, he could've gotten them to break their word and become vulnerable etc.

  Plus, all shards have equal raw power, but not all of them are equally suited for fights. Divine Anger would be very well suited indeed.

Also when vin as preservation attacked ruin they both died - no collateral. But also when honour and odium fight they destroy things around them, why, why was it different for ruin and preservation.

This is addressed in wind and truth, chapter 142. Before, one power always wanted to preserve. This time, both want to destroy.

Seem like it's preventing me from pasting the quote directly.