I am confused when the spellings "tion", "sion", and "cion" are used in words that contain the "shun" sound.
Are there any rules to help me understand when to use the correct spelling in a word?
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Here are the main guidelines to choose the right suffix among -sion, -tion, and -cion. The first two are the more common while -cion is actually quite rare. (Oxford Dictionaries Online)
Words ending in -sion
If the ending is pronounced as in confusion, then it should be spelled -sion. Here are some examples: collision; division; revision; persuasion; explosion; decision;seclusion.
When the ending comes after an -l, it's always spelled -sion: compulsion; revulsion; expulsion; emulsion; propulsion.
When the ending follows an -n or -r, it's often spelled -sion, especially if the word is related to one that ends in -d or -se. For example: immersion (from immerse); comprehension (from comprehend). Here are some more examples: aversion; conversion; apprehension; diversion; extension; version.
Nouns based on words that end in -ss or -mit always end in -sion: permission comes from permit and discussion comes from discuss Here are some more examples: commission; expression; aggression; admission; succession; impression; emission.
Words ending in -tion
If the ending is pronounced as in station, then it's spelled -tion. For example: addition; duration; nation; solution; ambition; edition; caution; position.
If the noun is related to a word ending in -ate, then the ending will be -ation, e.g. donation (from donate) or vacation (from vacate). Here are some more examples: accommodation; location; creation; rotation; education; mediation.
If the ending comes after any consonant apart from -l, -n, or -r, then the ending is spelled -tion: action; connection; reception; affection; interruption; description; collection; infection; deception.
After -n and -r, the ending can be -tion or -sion. It's more likely to be -tion if the word's related to another one that ends in -t or -tain, e.g. assertion (from assert) or retention (from retain). Here are some more examples: exertion; distortion; abstention; invention.
Words ending in -cion
There are just two common nouns that end in -cion: suspicion and coercion.