Should I avoid teaching the TH to my student since he has a frontal/interdental lisp? Would it be confusing? Or should I teach it before working on the S?
Other than perhaps the THs being a bit more interdental than ideal with orofacial myology disorders, the causes of TH errors are among the sound errors that are rarely related to myo. Usually, they are related to younger aged children who are not yet expected to produce the sound or perhaps related to certain cultural groups or from certain locations where F is substituted for voiceless TH while D might be substituted for voiced TH.
To teach S and TH simultaneously has not proved to be a good choice in my clinical experience. Because their interdental S and a TH are similar, the client tends to “overuse” one of them when they are taught around the same time. For example, to say “Thank you Scottie for my thick soft blanket,” they are likely to overcompensate and instead produce “Sank you Scottie for my sick soft blanket.” Or, “Thank you Thocttie for my thick thoft blanket.” While it is natural to over substitute during the learning process, teaching both at the same time would be counterproductive and confusing to the client.