The rule for determining whether or not to add the ‘t’ before ‘ch’ is: Add ‘t’ before ‘ch’ after a short vowel. For example, the following words all have a short vowel immediately in front of ‘ch’ and therefore require the silent ‘t’ – catch, fetch, stitch, notch, hutch. In contrast the following words either don’t have a short a vowel at all or don’t have a short vowel in front of the ‘ch’ and therefore do not require the silent ‘t’ – beach, ranch, lurch.
As is often the case in English there are exceptions to this rule. The most regularly occurring of these are in the following sentence: Which man is much too rich to have such a horrible sandwich. However, there are quite a few other words that ‘break’ the rule – attach, bachelor, duchess, etc.
So, what is the history behind adding in the silent ‘t’? There are two parts to answering this question. Firstly, you need to look at the history of the letter combination ‘ch’. The digraph was first used in Latin for the Greek letter chi (pronounced ‘kie’ so it rhymes with ‘pie’). In Old English, the /ch/ sound was represented by the letter ‘c’, so for example, ‘chin’ and ‘kin’ were both spelled the same way, either ‘cinn’ or ‘cynn’, depending on the region. Words today in which the ‘c’ is pronounced /ch/ have usually been ‘borrowed’ from another language (e.g., cello or cappuccino from Italian).
To make English even more confusing, the ‘ch’ can also be pronounced as /sh/, most commonly for words of French origin (e.g., chef, chauffeur), or /k/, most commonly for words of Latin origin (e.g., chorus, choir).
The next part of the puzzle is to understand why the silent ‘t’ is included in some words. This relates to the rule ‘double the next consonant to keep the vowel short when there is only one consonant after the vowel and the next syllable begins with a vowel’ (e.g., butter, hitting). It is very cumbersome to write double ‘ch’ (cachching), so it was decided in the 15th century to add a ‘t’ instead of doubling the ‘ch’.
Scragg, D.G. (1974). A history of English spelling. Manchester University Press: NY.
Upward, C., & Davidson, G. (2011). The history of English spelling. John Wiley & Sons: NJ.
Venezky, R., L., (1999). The American way of spelling: The Structure and Origins of American English orthography. Guildford Press: NY.