Which of these is correct?
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Mary and i like to read books.
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mary and I like to read books.
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mary and i like to read books.
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Mary and I like to read books.
Option D is correct because it uses proper capitalization. 'Mary' is a proper noun and must be capitalized, and 'I' is always capitalized as the first-person pronoun. Options A, B, and C are incorrect because they fail to capitalize either 'Mary' or 'I'. Capitalization rules for proper nouns and the pronoun 'I' are fundamental in English grammar.
Sentences always begin with a capital letter, and the pronoun "I" is capitalized wherever it appears, not just at the start of a sentence. "Mary and I like to read books." satisfies both rules correctly. The other options fail because they either capitalize the sentence but leave the pronoun "i" lowercase, or fail to capitalize the sentence's first letter.