Why does pronunciation of spells matter?

Question

Obviously, spells in Harry Potter can be cast nonverbally as well as verbally. In the first year Charms class, they haven't gotten up to nonverbal spells yet. But as they are trying to cast Wingardium Leviosa, Hermione tells Ron that the reason he can't cast the spell is because he's pronouncing it incorrectly. If the spell can be cast nonverbally, and therefore with no pronunciation at all, why should pronunciation matter for the verbal spell?

If you can cast spells by only thinking something, why does the pronunciation matter if you talk while you think it?

Answer 1

As I see it (this is just my interpretation as a reader), mastering spells seems to have levels. Much like Beginner, Novice, Advanced, Legendry, etc:

Beginner level:

Beginner stage. Mostly the first, second and third years. They are in the learning phase. And they need to perfect their wand movement and pronounce the spells properly in order to execute them well.

Novice and advanced level:

The seniors. A very wide band. Most of the people in the magical world fall in these levels. Easy-to-execute spells and charms can be done with the swish of the wand.

Like, for example, Mrs. Weasely sets the washing process of the utensils with a small flick of her wand, etc. Prof. Lupin making the kettle brew coffee with a flick of his wand.

Mostly the mundane work can be done with a small flick or minimal effort. McGonagall waves the wand very lazily(to conjure up plates) when Harry and Ron had to have their feast in her office.

Advanced level:

Doing advanced level magic with just a flick of a wand. People like Mc Gonagall, Dumbledore, Voldemort, etc.

For example, Dumbledore conjured up hundreds of sleeping bags with just a swish of his wand (in PoA), and so does McGonagall.

If there's a God Level, then it should be Dumbledore and Voldemort.

(However, insanely powerful spells(or curses) like Avada Kedavra takes a huge effort to pull off non-verbally, even for Voldemort).

For example, this quote from the Order of the Phoenix shows how powerful Dumbledore and Voldemort are, even with non-verbal spells:

"Dumbledore flicked his own wand: the force of the spell that emanated from it was such that Harry though shielded by his golden guard, felt his hair stand on end as it passed and, this time, Voldemort was forced to conjure a shining silver shield out of thin air to deflect it"

Answer 2

Yes pronunciation maters.

“STUBEFY !” Neville shouted again, pointing his wand at each Death Eater in turn, “STUBEFY ! STUBE —”

At this point we know neville is competent enough to use stupefy but because of his broken nose he cant pronounce it right, and nothing happens.

I believe that when producing non-verbal spells the caster is also THINKING the incantation in their head. will update with a link to confirm .

"Pointing his wand at nothing in particular, he gave it an upward flick and said Levicorpus! inside his head. "Aaaaaaaargh!" There was a flash of light and the room was full of voices: Everyone had woken up as Ron had let out a yell. Harry sent Advanced Potion-Making flying in panic; Ron was dangling upside down in midair as though an invisible hook had hoisted him up by the ankle.

@davidbanner, thanks for finding this, as we can see you say the word in your head, so if you cant pronounce it properly verbally, your probably will fail to cast in non-verbally since you have the wrong pronunciation in your mind.