Japanese speakers
Mistake: Pronouncing it as "FAINDO ZEMU" with a full d release and full [ðɛm].
Why: Katakana English preserves each letter.
Fix: The d of "find" elides before the [ð] of "them", and "them" reduces to [əm]: [ˈfaɪnəm]. Or with [ð] preserved: [ˈfaɪnˈðəm]. Both natural.
Korean speakers
Mistake: Producing clear [d] and full [ðɛm].
Why: Korean has no [ð] and prosody preserves boundaries.
Fix: d elides before [ð], and "them" further reduces: [ˈfaɪnəm]. In fast speech, sounds like "findum" as one word.
Spanish speakers
Mistake: Producing both [d] and [ð] with Spanish weakening.
Why: Spanish has [ð] as allophone.
Fix: The d of "find" elides; "them" reduces heavily to [əm]: [ˈfaɪnəm]. Same pattern as "ask them" [ˈæskəm], "tell them" [ˈtɛləm].