When I start my pronunciation unit on the “th” sounds, half of my class sounds like snakes and bees.
That’s because many languages don’t have theta (/θ/) or eth (/ð/) sounds, so those sounds come out as /s/ and /z/ respectively. If you do a speaking exercise with a lot of words with “th”, you’re going to hear “sssss” and “zzzzz” until you start helping certain students improve their pronunciation.
Since both of these sounds are represented with a “th” in English writing, you can teach your students the symbols – θ for unvoiced, and ð for voiced (these are real letters, just not in English. You’ll find θ in modern Greek and ð in modern Icelandic – pretty cool!)
For Arabic speakers learning English, this will make sense to them since they have two different letters for these two sounds:
ث = θ (unvoiced) and ذ = ð (voiced)
Arabic speakers will have no problem pronouncing these sounds since they have them in their language. But they might have trouble choosing which sound to use when they see a “th” in writing, so they can still get something out of a lesson on these two sounds.
Let’s consider a few more things when you’re teaching the “th” sounds.