stem-changing verb: A verb that requires an internal change in the vowel before the -ar, -er, or -ir infinitive ending in all the singular and third-person plural forms of certain tenses.

subject pronoun: A pronoun used as a subject.

subjunctive: A grammatical mood indicating uncertainty, desire, supposition, hypothetical and theoretical situations, or impersonal opinions.

subordinate clause: A part of a sentence containing a subject and verb that can’t stand on its own but describes something in the larger sentence.

suffix: Letters added to the end of a word that change its meaning, give it a different grammatical function, or form a new word.

superlative: A form of an adjective or adverb which indicates that one thing has some feature to a greater degree than anything it is being compared to. Example: greatest, most honorable

syllable: A word or part of a word pronounced with a single sound, usually consisting of a vowel and one or more consonants.