* Nouns: Gender (masculine/feminine), number (singular/plural), and case (although Spanish case is less prominent than in some other languages like Latin).
* Articles: Definite (el, la, los, las) and indefinite (un, una, unos, unas) articles agree in gender and number with the noun they modify.
* Adjectives: Adjectives agree in gender and number with the nouns they modify.
* Verbs: Spanish verbs are highly conjugated, changing form to indicate tense, mood (indicative, subjunctive, imperative), aspect (perfect, imperfect), person (first, second, third), and number (singular, plural). This conjugation is significantly more complex than in English. There are different verb conjugations based on the verb's ending (e.g., -ar, -er, -ir verbs).
* Pronouns: Personal pronouns (yo, tú, él, ella, nosotros, vosotros, ellos, ellas), possessive pronouns (mi, tu, su, nuestro, vuestro, su), demonstrative pronouns (este, ese, aquel), relative pronouns (que, quien, cuyo), and interrogative pronouns (quién, qué, cuál) all have specific uses and grammatical rules.
* Prepositions: Prepositions (a, de, en, para, por, con, etc.) show relationships between words in a sentence.
* Sentence Structure: Spanish generally follows a Subject-Verb-Object (SVO) sentence structure, but there are variations and exceptions. Word order can be more flexible than in English, but changing it can alter emphasis.
* Adverbs: Adverbs modify verbs, adjectives, or other adverbs.
* Conjunctions: Conjunctions connect words, phrases, and clauses (e.g., y [and], pero [but], porque [because]).
While there aren't single, concise rules that cover everything, Spanish grammar textbooks and resources comprehensively detail these rules and their nuances. The complexity arises from the interactions between these different grammatical components. Learning Spanish grammar involves mastering these interconnected rules and their exceptions.