I am reading a book which contains phonogram rules for English spelling. For the multiple letter phonogram sound of -ti, it says
-tall /sh/ used only at the beginning of any syllable after the first one as in the word 'partial'" So the first syllable here is "par" and the second is "ti".
So to say, the -ti is pronounced as /sh/ here. But what about the words which fits this rule pronounced differently, such as; optimal, optical and etc.?
Okay, I guess I've got it. It says
at the beginning of syllable
so, "-ti" has to be a part of syllable but not a syllable on its own. When it is syllable on its own as in;
op-ti-cal
op-ti-mum
op-ti-mal
then it is pronounced as it is written (ti).
But in the words such as;
par-tial
op-tion
di-rec-tion
"ti" represents the beginning of the syllable 'tion'. So it fits the rule and therefore it is pronounced as /sh/.
I hope it will be helpful for others. I see that I just missed the point here.
I don't entirely agree with the rule you mentioned. There must be a vowel after the ⟨ti⟩ in order to produce /ʃ/. The reason the ⟨ti⟩+V (V being a vowel) becomes /ʃ/ is palatalisation.
In simple words, when ⟨ti⟩ is followed by a vowel, the vowel becomes a palatal glide /j/ which pulls the place of articulation of the /t/ towards the hard palate (or post-alveolar region) and makes it /ʃ/.1
This change has happened historically in many words, for instance, inertia, nation, partial etc.
/sh/ used only at the beginning of any syllable after the first one as in the word 'partial'" So the first syllable here is "par" and the second is "ti".
This is incorrect and misleading. Suppose there's a word op.tim; the ⟨ti⟩ is at the beginning of a syllable after the first one, but it can never be pronounced /ʃ/ because there is no vowel sound after it.
And that's the reason why ghoti cannot be pronounced fish.
1. The ⟨ti⟩ used to represent [ts] in Latin which was simplified to [s] before those words entered English and as we know, [s] and [j] coalesce to [ʃ], that's why ⟨ti⟩+V represents /ʃ/.
