English spelling and reading can sometimes present challenges due to some of the complexities of the grapheme-phoneme correspondences. Several graphemes (spelling units) represent more than one pronunciation and a single pronunciation can be written using different graphemes. However, the complexities of the grapheme-phoneme correspondences provide us with valuable information about words. How a word is written may provide glues about where (language of origin) it was borrowed from or how long it has been in the language.
I am going to use the grapheme <t> to illustrate this point. The grapheme <t> is pronounced in three different ways: /t/, /ʃ/, and /ʧ/ as in <time>, <nation>, and <nature>. (Remember, the angle brackets <> are used to represent spelling and the slashes // represent pronunciation.) The idea that the same grapheme might represent several different pronunciations might seem overwhelming at first, but a careful attention to the words in which a particular grapheme appear may reveal some order.
The default pronunciation for the grapheme is /t/. A default grapheme for a particular pronunciation means that the grapheme can be pronounced that way in any part of a word since that is the basic grapheme for that pronunciation. However, a careful look reveals that <t> pronounced as /ʃ/ (usually represented with the basic grapheme <sh>) as in <nation> does not occur in an initial position in a word. To put it simply, <t> is not pronounced the same as <sh> at the beginning or end of any word. In the same way, <t> is not pronounced the same way as <ch> at the beginning or end of a word. Most of the time, the best way to fully understand the operation of the grapheme-phoneme correspondences is to gather an evidence bank of words in which they appear and study them.
For <t> to correspond to /ʃ/ like <sh> it has to be in the middle of a word and followed by either <i> or <e> and another vowel letter as in <nation, nutrition, nutritious>. Also, <t> for /ʧ/ like <ch> always occurs in the middle of a word and before a <u> as in <nature, picture>. The linguistic term that describes the process that creates such pronunciation is called assibilation. Words in which <t> correspond to these two pronunciations tend to be of Latin origin which came into English either directly from Latin or through French.
A sure way to make sense of the grapheme-phoneme correspondences is through the use of a matrix. This is one of the reasons why understanding how morphology operates in English is so useful. It stops children (and sometimes even adults) from spelling words based on how they pronounce them. To further illustrate this idea, I am going to present you a matrix of the base <part> where the grapheme <t> assumes all the three different pronunciations in the different derivatives of the base.
See how many words you can form from the matrix using word sums. I will show you a few examples and note how the pronunciation of the <t> in <part> changes based on what suffixes are added to it.
De + part –> depart
De + part + s –> departs
De + part + ed –> departed
De + part + ing –> departing
De + part + ure –> departure
De + part + ment –> department
Part + ly –> partly
Part + i + al –> partial
Have you noticed how the <t> is pronounced in <depart>, <departure>, and <partial>? You can study the phonemic correspondences of <t> and other graphemes using the matrix as a starting point. This reinforces the fact of morphemes remaining stable even though how they are pronounced change based on the phonological environments in which they appear. In this way, it becomes easy to understand why there is no <sh> in <partial> and no <ch> in <departure>. Such a matrix also allows you discuss how stress affects pronunciation since unstressed syllables such as what you have in <partial> often pose problems for children in both reading and spelling