I just noticed how proper pronunciation of a spell is more difficult than I had previously thought, I'm a non-native English speaker and I find it really hard to pronounce certain English words such as sixth, world, three, ninth, Antidisestablishmentarianism, Isthmus, rhythm and others.

If you're a native English speaker, think of how difficult it is to speak Russian fluently and without your accent. And how would you know how to pronounce it if you've never heard Russian before? So learning incantations straight out of a book without any guidance or experience is extremely dangerous.

In my opinion, the main reason why students can't cast certain spells is because of improper pronunciation.

 

They practice the pronunciation in class, but I suspect they learn how the incantations are supposed to be pronounced as they go. Latin words generally alternate between vowels and consonants, eliminating the isthmus, sphinx, sphere, and other wacky sounds you get when you run a bunch of consonants together. In addition to a lack of syllabic blowjobs you have to perform to get the words to sound the way they're supposed to sound, it's pretty easy to know in your head how they're supposed to be pronounced because every letter makes exactly one sound. If you've heard it, you can spell it; if you've seen it written, you know how it's pronounced.

 

Thank you for your addition, but don't you think that despite all that, some students still can't pronounce the spell correctly and that is why they fail? It's not a lack of power but of skill.

 

The only mention of this is in the first book where Ron has a bit of difficulty due to his West Country accent on like the second month. No one else is mentioned to have any difficulty and Ron never has a problem with it again. That same day, even, he uses the same spell he had difficulty pronouncing to defeat the troll. I imagine students come to Hogwarts not knowing how to pronounce things, but evidently they pick up on it easily enough.