interchangeable – adj. capable of being used in place of each other

conjunction – n. a word that joins together sentences or clauses

polite – adj. socially correct or proper

doubt – n. a feeling of being uncertain or unsure about something

introduce – v. to bring something into use or operation for the first time

circumstance – n. a condition or fact that affects a situation

clause – n. a part of a sentence that has its own subject and verb