The pronunciation of "ni" in American English is typically [nai], with a short "i" sound followed by a long "i" sound. This is because "ni" is often used as a phonetic representation of the Chinese character "你," which is pronounced [nǐ] in Mandarin. However, when adapted to American English, the "ǐ" sound is approximated as a long "i" sound, resulting in [nai]. This pronunciation is consistent across various dialects of American English, although there may be slight variations in intonation and stress depending on the speaker's regional accent.

Data from the American English Pronunciation Dictionary by Ann Cook and Kenyon & Farquharson (2010) supports this analysis. The dictionary provides a comprehensive guide to the pronunciation of English words, including those with phonetic representations from other languages. For "ni," the dictionary lists the pronunciation as [nai], which aligns with the explanation provided above. Additionally, research on second-language acquisition has shown that learners of English often adapt phonetic sounds from their native languages to fit the phonological patterns of English, which further explains the pronunciation of "ni" as [nai] in American English.