The suffixes -tion and -sion are both used to create especially nouns from verbs to describe an action, process, practice, or the result thereof. They mainly of the same suffix -ion although we consider them individually for learning purposes.
There is also a third version, -cion, but this only occurs in only two word: coercion from the verb coerce, and suspicion, from the verb suspect.
When to write ‘–sion’
There are fewer words that end in sion than in tion. The sion is often formed from verbs which end with -d, -de, -se, -t.
This is why we write collision because its verb is to collide, comprehension (from to comprehend), infusion (from to infuse), persuasion (from to persuade), explosion (from to explode), television (from to televise) and decision (from to decide).
Also, when words end in ‘-ss’ the rule is just adding ‘–ion’ for example discuss takes discussion, confess and confession, obess and obsession, possess and possession, express and expression, recess and recession, profess and profession, process and procession.
The suffix ‘-ssion’ also takes a word that come from verbs which end in ‘mit’ for example admission from admit, permission from permit, commission from comit, remission from remit, transition from transmit, submission from submit.
When the ending comes after an -l, it’s always spelled -sion: compulsion; revulsion; expulsion; emulsion; propulsion.
When to write ‘-tion’
If the noun is related to a word ending in -ate, then the ending will be -ation, for example donation from donate, vacation from vacate, accommodation from accommodate, location from locate and creation from create.
If the ending comes after any consonant apart from -l, -n, or -r, then the ending is spelled –tion as we have action from act, connection from connect, reception, affection, interruption, description, collection, infection and deception.
After the above rules, you should be able to note that the spelling for a word that comes from a verb to invent is invention not invasion as some learners confuse them.
The noun invasion takes the sion because it comes from the verb to invade which corresponds to the rules we have seen above.
Similarly, we write transition not transission because the word comes from transit which ends in -it and write transmission because the word is related to a verb transmit which ends in -mit.