Spelling is a difficult aspect of English - for native speakers and non-native speakers alike. ESL students, encountering a word for the first time in an oral context, will often be unable to predict its spelling. Conversely, if they meet it for the first time in writing, there is no guarantee they will be able to pronounce it correctly.
enough / cough / although / through / plough
red / read - pour / poor / paw / pore
Contrast this with German or Turkish, where any given speech sound has an almost completely predictable written form.