Isthmus is pronounced IS-məs (IPA /ˈɪsməs/). Drop the 'th' completely — say 'IS-mus' in two beats.
IPA: /ˈɪsməs/
Syllables: IS · məs
Common ESL mistake: Learners say 'isth-mus' pronouncing the 'th' cluster fully.
Tip: Drop the 'th' completely — say 'IS-mus' in two beats.