You learn to speak a language by hearing and talking it (seeing and talking in the case of sign language). You learn to form the sounds and attach them to meanings based in context. If everytime you are given grapes and I call them sweets you will know no different until you interact with others.

To learn to read and write you have to be able to see and recognise shapes, associate those shapes with words and learn to replicate the shapes to be able write. Then you have to learn to put the shapes in a certain order to help them make sense. For example words go in a certain order with punctuation to help make sense. For example big bad wolf is right but bad big wolf is not but does it really makes sense as to why?

This is why it's easier to speak not write a second language and why people with large vocabularies can be illiterate