English spelling: Adding /ʃǝn/ (or /ʒǝn/) to base-words and changing from -tion to -sion

One tricky problem with English spelling is the variation in the nominalization suffix often represented in discourse as ‘shun’, mainly between -tion and -sion. Current ELT textbooks have generally not discussed rules for its spelling. However, following online resources, some basic rules are in current debate, with two main schools of thought, each falling in line with one of two approaches that can be called the ‘word-based model’ and the ‘base-word model’. In this article, I show the base-word model to be preferred, determine the actual suffix and its underlying form, and elaborate on base-word ending clues to yield a general synchronic rule for changing from -tion to -sion, albeit with exceptions.