The word for "refuse" in "refused" is "refuse." This is a straightforward case of the past tense form of the verb "refuse," which means to decline or to reject something. The past tense form is formed by adding the suffix "-ed" to the base verb "refuse." This pattern is common in English, where many verbs form their past tense by adding "-ed" to the base form.

The reason for this is rooted in the grammatical rules of English verb conjugation. English verbs typically follow a set pattern for forming their past tense and past participle forms. For regular verbs, the past tense is formed by adding "-ed" to the base form. For example, "walk" becomes "walked," "run" becomes "ran," and "refuse" becomes "refused." This system is consistent and predictable, making it easy for speakers of English to form the past tense of most verbs.

Data from the American English corpus, such as the Corpus of Contemporary American English (COCA), supports this pattern. The COCA is a large collection of American English texts that provides a wealth of information about word usage. According to the COCA, the past tense form "refused" is used significantly more frequently than the base form "refuse" in contemporary American English. This indicates that the past tense form is the more common and preferred form in everyday language use.

In conclusion, the word for "refuse" in "refused" is "refuse," and this is due to the grammatical rules of English verb conjugation, which dictate that regular verbs form their past tense by adding "-ed" to the base form. The frequency of use in contemporary American English, as evidenced by the COCA, further confirms the prevalence of the past tense form "refused.