pronunciation - What rules of English allow the first t in “patient” to make an sh sound?

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?

Answer

For the second time today, I feel reminded of ghoti. There's an interesting essay linked from that Wikipedia page, and I humbly direct you to it: "Hou tu pranownse Inglish".

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:

14. ci or ti becomes $ before a vowel: gracious = grä$@s, nation = nä$@n.

[...]

Rule 14 shows another reason ghoti is a fraud: ti only fricativizes when it's followed by a vowel.

In fact, the Wikipedia page itself has this linguistic analysis:

The /ʃ/ sound can be spelled eleven ways in English: shirt, sugar, chute, action, issue, ocean, conscious, mansion, schwa, anxious, and special. Some speakers of English do not use /ʃ/ in all of those words. For example, issue may be pronounced as /ˈɪsju/ by some speakers of British English. This phoneme is spelled ti only when the ti comes before a vowel in certain suffixes, as in nation and initial. Thus, this phoneme does not actually occur at the end of English words with the spelling "ti".