What rules of the English language allow the first t in patient to make an sh sound?
Why is it
/ˈpeɪʃənt/ and not
/ˈpeɪtənt/? Are there any other words where t behaves in this way?
If you don't want to read the entire thing (but I recommend it!), the rule you are looking for is number 14 on that list:
In fact, the Wikipedia page itself has this linguistic analysis:
The reason something pronounced [ʃ] ("sh") would ever get the spelling "ti" is because of palatalization. Basically, the "io" diphthong contains a palatal consonant [j] ("y" sound), which, in certain cases, pulls the place of articulation of other consonants towards it (e.g. t->ʃ).
The palatalization process is no longer active in these words spelled with "ti" (nowadays it is just 100% /ʃ/), but palatalization is active in some British dialects, for pronunciations of words like "Tuesday": /tjuzdeɪ/ ("Tyoozday") comes out as [tʃuzdeɪ] ("Choozday").
(Note that in most dialects of American English, we just say [tuzdeɪ] ("toozday"), because we don't have a palatal consonant in that context at all.)
What makes you think it is a rule?
Languages are not designed to a set of rules laid down by a designer. That idea was tried with Esperanto, and it doesn't work.
Language is organic. It grows and shifts with the tides of time and change.
English is an evolving entity, and many of its seemingly strange rules, only exist in the wishful mind of non-native speakers.
I don't know of any native speaker who has ever thought much about Engiish spelling, other than "I am good at spelling", or "I am not very good at spelling".
Is English spelling hard for a non-native speaker? Yes, but that is just the way it is!
Is English spelling hard for a native speaker? Not especially, but it is a learnt art; learnt over many years.
Many of the quirky spellings come from the pronunciations of the French, German, Latin, Gaelic, etc. root languages... and believe it or not, there is an underlying rhythm; a series of patterns based somehow on the different root languages.
What is the rule? you ask... You have asked the wrong question.
Perhaps a better question is how do I deal with all the unusual spellings?
If you want to learn English, well that is how every native English speaker has learnt them; by memorizing them...
It is simply a parallel discipline! ... jus like speaking is discipline.
As as for all that utter nonsense about ghoti, it is an absolute waste of time! You are missing the point, and wasting your time with what non-native speakers see as conflicting spellings.
Slowly, slowly, the patterns reveal themselves.. (even if it is unconsciously)
PS: Mathew.. I hadn't noticed that you were in the US... I'm so used to dealing with non-native speakers in language-exchange environments, where many seem to think that Engish spelling is based on a few simple logical rules... Oh well, my answer may be more appropriate for ESL readers of this question..