Magical Hypothesis

Understanding that the surface held no imminent dangers, Kai continued his resource exploration, dispatching tendrils into the depths. Simultaneously, he delved into formulating theories about the relationship between Magic and Mana.

So Mana is unrefined essence, and Magic is Mana that has been distilled and transformed into another form of energy, and to control Magic one needs to either chant or imagine the shape and properties of the magic. I'm guessing chants are cognitive aids that help people to imagine the form their Mana has to turn into, while visualization can skip this step.

Kai was deep in thought, he had come up with a believable hypothesis after his observations on how his Mana and Magic acted. It didn't take long before he'd managed to efficiently conjure Magical fireballs and ice spears at a single thought, sending them flying into the lake.

The speed of the projectile is determined by the amount of Mana allocated into the "speed" part of the spell. Kai added as he watched a similarly large fireballs launch at different velocities.

Immersing himself in these magical experiments, Kai found that time flew by. When he finally looked up, he found the sky awash in a rich amber glow, signaling the sun's descent.

Reflecting on his accomplishments, he thought back on his progress so far. Firstly, he'd conjured Magic in different forms. Large pillars, spikes, circles, bullets, and weapons of fire were all successful, though to make them solid he'd needed to condense the Mana, which he'd found was possible to do, allowing him to stuff crazy amounts of mana into small forms like bullets.

Kai converted Mana into water, molding it into a spear shape, and launched it around like a maniac. It embedded itself onto solid surfaces before losing its shape and falling onto the ground as normal water, it remained in as spear-like shape in the water even longer. he'd found out that the more Mana he put into the "Shape" aspect of his creations allowed a more consistent and stable form, allowing his water spears to remain as spears longer.

Earth was a tricky one. Kai found out that the conjuration of Earth element aligned objects like dirt took a much heavier toll than its other elements. He'd discovered it was easier and cost much less Mana to manipulate already present Earth, though this limited the versatility of the element but was also the most cost-efficient.

The Wind element was also a hard element to work with. It was difficult for Kai to imagine the wind, after all, the only way we could "see" wind moving was the way things fluttered in the wind, and the drawings allowed people to imagine the wind a little better. He'd managed to create slight breezes that caused the trees to sway a little, however, that was the limit of what he could do without any chants.

O Sovereign of Wind Zephyr, lend your might to this mortal coil and unleash your tempests! Gale

Kai made up a chant, basing it off of a Greek God that he'd read about in his past life. 

"Woosh"

A violent gust of wind erupted from Kai's body, causing his surroundings to sway incessantly. Seeing this, Kai was bummed because he'd have to say a chant every time he'd wanted to cast a decent Wind spell.

Moving on, Kai managed to link different spell names to the respective forms and properties of Mana. For example, whenever Kai cast a Fireball, he didn't need to imagine his Mana converting into a small ball of fire as intently as before, as though his mind unconsciously remembers the "Formula" of the spell; Only requiring him to imagine what he wanted the spell to do. The efficiency only increased with every time Kai used the spell, not taking him long before he was able to cast fireball effortlessly.

This is what Kai spent most of his day doing, spamming spells and recovering his Mana after expending most of it. Kai could feel his Mana expenditure, however, his body wouldn't warn him when it got low and would only do so when it literally ran out. Causing Kai to be extra cautious not to use up too much Mana.

It didn't take long for Kai to begin working on Hybrid Spells - Spells that use more than one element. Combining Fire and Water elements, he'd conjured them using the same amount of Mana at the same spot the first time. Needless to say, the contrasting elements rejected one another and caused the ball of Water and Fire to burst apart, the fire being instantly doused and the Water bursting apart.

Kai tried again, this time making the Fire use double the amount of Mana used. This time, the Fire caused the Water to start boiling. Repeating this experiment, he'd found the best ratio of Fire Mana to Water Mana - 5 to 1. This combination of Elements created instant Steam. He also discovered that the temperature of the steam was directly proportional to the temperature of the Fire. 

The more Mana Kai dedicated to the Fire, the higher its temperature. Seeing this, he added half his mana to the equation. The results were... quite shocking. The steam's heat was terribly high, and although its volume was small due to the volume of the (Mana-concentrated) water, Kai could see it visibly distorting the air around it. Fortunately, the steam dissipated upward, leaving the local ecosystem unharmed.