Japanese speakers
Mistake: Pronouncing it as "TAAN OFU" with a katakana schwa vowel and a full [u] on "off".
Why: Katakana English inserts vowels and cannot easily produce the English [ɜːr] or the tight [ɔːf] ending.
Fix: Link the n of "turn" onto the o of "off": [ˈtɜːrnˈɒf]. Say it as one word, "turnoff". The [ɜːr] vowel in "turn" is rhotacized (r-colored) in GA.
Korean speakers
Mistake: Producing a glottal reset before "off" and a heavy final f release.
Why: Korean prosody adds glottal onsets to word-initial vowels and treats fricatives with strong friction.
Fix: Let the n link straight to the [ɒ]: [ˈtɜːrnˈɒf]. Keep the final f short and light, not extended.
Spanish speakers
Mistake: Producing the r of "turn" as a rolled or tapped Spanish r, and voicing the final f to [v].
Why: Spanish r is a distinct tap/trill; English r is a light approximant. Spanish speakers also sometimes voice word-final consonants.
Fix: English r in "turn" is [ɹ], not [r] or [ɾ]. And keep f voiceless: [ˈtɜːrnˈɒf]. In RP, the r may be silent: [ˈtɜːnˈɒf].