It goes without saying that most Brazilian English language learners (BELLs) (and teachers, especially novice ones) have a hard time when they attempt to pronounce the dental fricative sounds /θ/ and /ð/, aka “th sounds.” First, it is hard because these sounds are not part of our Brazilian Portuguese language inventory. Second, students think that either the /s/ or the /f/ sound will replace the /θ/ and that the /z/ or the /v/ sound will do the same for /ð/. However, we can do some things to assist our BELLs and our fellow novice teachers.
One of them is to avoid using the old-school sentence (the one my first English language teacher used circa 1992):
“Sopinha de maçãzinha amassadinha, three.”
Even though the teacher asked me to say the /s/ as a dental fricative sound, i.e., by putting my tongue between my upper and lower teeth, it didn’t work for I tended to say something like “sree” /sri:/ and not “three” /θri:/. Thus, even before coming up with another sentence to address this issue, it is paramount to say that overcoming a pronunciation obstacle is a lot like going to the gym and breaking a sweat. You have to work the facial muscles and your vocal tract to produce those articulations, which do not belong to our mother language. Moreover, showing BELLs and novice peers the points of articulation for both /θ/ and /ð/ in words like “think” /θɪŋk/ and “mother” /ˈmʌðər/ makes a world of difference before the workout above.
Once they become aware of such articulations, it is time to emphasize that their interlocutors, i.e., native and non-native speakers of English, perceive the difference between /f/ and /θ/, and /v/ and /ð/. This way they are going to be able to overcome that fallacy that there are no differences between a labiodental and a dental fricative. If there weren’t such differences, I could say things like “I have free brothers” when referring to the number of siblings I have, and that is not the case.
What is more, to emphasize such differences, we can bring tiny mirrors to class to have students articulate both the dental and the labiodental sounds, or even ask them to hold down their lower lips when producing the dental ones. However, it is important to remind both learners and novice teachers not to leave the classroom holding down their lips when carrying out conversations in English. Trust me, it had already happened to some of my students.
Last but not least, after being in the ELT field for over 18 years, I have put together a sentence which can truly assist my students when practicing the articulation of dental fricative sounds:
“Fui à feira comprar frutas e verduras com o Valter e o Frederico e vimos a Viviane.”
Thus, all I do is ask them to articulate the /f/ as /θ/ and the /v/ as /ð/. Once students feel more confident swapping the articulation, I ask them to say words like “three,” “think,” “through,” “though,” “the” and the like. In the beginning, they can’t help but laugh, but the results I got so far are not a little better, they are much, much better compared to those from the maçãzinha.