The English alphabet has 26 letters, which we might teach through a nice little song. But we produce a lot more than 26 sounds when speaking English (it’s closer to 40, depending on the dialect), so why do we limit our alphabet?
The ABCs are comprised of the individual letters that are uniquely written or typed. That’s obviously fundamental to our written language and should be taught very early in education. It’s worth noting, however, that some languages include digraphs (or other multigraphs) in their alphabets. Digraphs are combinations of two different letters that produce a unique sound (or more accurately, a sound that neither of those two letters would make on its own). Common digraphs in English include:
There are plenty more, but new learners are likely to encounter these four early and frequently. If you hear/say these sounds, you need to write the words using these letter combinations; and – with some exceptions, especially with ‘ch’ – when you read words with these pairs, you can expect to produce those specific sounds to pronounce the words they’re in. In other words, digraphs function the same way as single letters, so perhaps we should teach them as a part of the alphabet.
After all, using ‘sh’ has nothing to do with either ‘s’ or ‘h’, except in their shapes. ‘sh’ really is a unique element of spelling and reading.
I’d imagine that most teachers would explain the ‘ch’ after teaching beginners the word ‘chair’ or ‘cheese’ or ‘chicken’, or some such word, or even multiple such words. They might even wait until students need to spell the word, which might come after speaking or even reading the word. I think we’re likely to teach it only when a problem crops up because we don’t really think about until then. But why not be proactive? Why not avoid confusion by teaching ‘sh’, ‘ch’, ‘th’, and ‘ng’ along with the rest of the ABCs?
Of course, you’ll need to decide what’s best to introduce to your students based on your curriculum. For example, you might want to teach ‘ph’ along with the rest because, even though it’s not terribly common, students will encounter it with ‘elephant’ or ‘phone’ in their first year. You also might need to adjust how you introduce the alphabet based on your students’ native tongue. If they already know the Latin alphabet, explaining letter combinations shouldn’t be too difficult for them to understand; on the other hand, if they have a very different alphabet, or even no alphabet at all, it might make sense to delay the digraphs until they’ve gotten a handle on our basic 26. Talk with other teachers in your country to decide what approach would work best in your classroom.
In any case, digraphs probably shouldn’t be ignored until your students have repeatedly mispronounced or misspelled them. I recommend teaching the most common letter combinations while or shortly after teaching the basic alphabet, which will better set students’ expectations.