Final answer:
The incorrectly capitalized word in 'My teacher's name is Mary ann Smith' is 'ann'. Proper nouns such as personal names should always have their initial letters capitalized.
Step-by-step explanation:
The word in the sentence that is incorrectly capitalized is 'ann' in the phrase 'My teacher's name is Mary ann Smith.' In English, proper nouns, including the names of individuals, should always be capitalized.
Therefore, the correct capitalization of the name would be 'Mary Ann Smith' where both the first and middle names are capitalized.
Capitalization rules dictate that we capitalize proper nouns, which include people's names, to distinguish them from common nouns. For instance, 'Anne Mansfield Sullivan' and 'Prof. Chang' are examples where proper nouns are correctly capitalized in reference to individuals.
Adherence to capitalization rules is crucial in writing to ensure clarity and proper acknowledgment of proper nouns, titles, and other specific terms.