When we were in primary school we learnt that words are made out of small buildings blocks, phonics. C... A... R.... Car, f...I....sh... Fish. For most words this works great, but theres a few problems.
One is the same sound can be spelt multiple ways c, k, CK. F, ph. There, their, there. Ect. And even though you're using the building blocks of phonics the words you make can be completely wrong.
You also have words that don't even try to phonics. Have for instance magic E means it makes an aaayy..(Fonzie) sounds, but it doesn't it makes an aahh... (Sip of tea) sound. One makes a w sound. Said should be pronounced s I D yet AI is making an eehh... Sound here. How should be pronounced haw as in 'Haw you! Whit you staring at you....' you get the idea.
As you can see even after following the rules it's very easy to get spellings wrong and what happens when we do? We're labelled stupid or dislexic. There's nothing stupid about using the building blocks of language and getting the wrong answer through no fault of our own. English is just weird, unlike math's 2 + 2 can equal four, but it can also for, or foor or even fore.
Sometimes, a lot of the time you just need to memorise the word as a picture. Like a Chinese character or hieroglyphics. Words like - Pneumonia, choir, Wednesday, Receipt, vegetable ect.