In written language, our thoughts and our speech are represented by visual symbols to convey the meaning of what we are writing and saying. In an alphabetic language like English, the 'visual symbols' are letters. These are combined together into words which have meaning for us. Sometimes these 'letters' correspond to our perception of the sounds we hear in the words we speak. Sometimes they do not.
In the 17th century spellings of English words were 'set in stone' when the first dictionaries were written. These spellings have not changed over time. It is because they have not changed over the years, that we can read books written in previous centuries.
In some languages each letter corresponds to a specific sound. Learning to read and write is fairly easy in these languages. But the English language has only 26 letters in its alphabet and 44 different sounds are standard in British Received Pronunciation. There is no single correspondence between each letter and each sound.
Sometimes a combination of letters corresponds to single sound e.g. 'ch' as in 'chip'. Sometimes the same letters correspond to a different sound, e.g. 'ch' in 'chemist' or 'ch' as in 'chef'.
Sometimes a single letter can represent more than one sound, e.g. 'a' in 'cat, father, baby'. Each 'a' corresponds to a different sound. The letter 'y' is pronounced differently in 'mummy, my, yet, gym' as well as being part of vowel graphemes in 'ay (day), ey (grey), oy (toy), ey (donkey)'. This makes learning to read and write in English very difficult. Sometimes we need to know the meaning of the word before we can decide how to pronounce it, e.g. 'row, row; wound, wound; beak, bread, break'
We can learn all the different pronunciations of the letters and letter combinations, and still this will not tell us to how to choose the correct pronunciation of a word without knowing what it means in the context of the written message. We need to relate the spelling of the word, i.e. its written symbols, to something else we already know, i.e. what the word means.
Let's consider letter symbols too. If we think of the first letter of the alphabet, 'a' or 'A, we can see straight away that it has two written symbols. (In actual fact 'a' has more than two symbols because children do not write the small letter 'a' in this particular type-font. They use the shape of 'a' as that in the Sassoon Primary Infant, Century Gothic or Comic Sans fonts.)
The first symbol is lower case on the keyboard, or a 'small' letter. The second symbol is upper case on the keyboard, or a 'capital letter'. The capital letter is only used at the beginning of a sentence or the beginning of a name, and its corresponding sound is the same as that of the small letter. We would pronounce 'cat, baby, father,' exactly the same when written in capital letters 'CAT, BABY, FATHER'.
So the capital letters do not correspond to different sounds and they do not add to our 26 letters in the alphabet.
If we pronounce the words 'car' and 'father', 'down' and 'about', 'form' and 'pawn', 'royal' and 'boil', 'girl' and 'curl', 'hair' and 'stare', 'year' and 'deer', we can hear the same sound for the vowels in each pair of words, but the spellings are different.
How do we learn which spelling goes with which sound in which word? We can only do that if we know the meaning of the word and we have this extra information to help us. The sound of the word alone will not tell us how to spell it. We need to visually memorise it also.