Sentence Variety
Complex sentences: https://www.khanacademy.org/humanities/grammar/syntax-sentences-and-clauses/types-of-sentences/v/complex-sentences-syntax-khan-academy
Compound-complex sentences: https://www.khanacademy.org/humanities/grammar/syntax-sentences-and-clauses/types-of-sentences/v/compound-complex-sentences-syntax-khan-academy
Practice with complex and compound-complex sentences: https://www.khanacademy.org/humanities/grammar/syntax-sentences-and-clauses/types-of-sentences/e/complex-and-compound-complex-sentences
Pronouns
Pronoun-antecedent: https://www.khanacademy.org/humanities/grammar/syntax-conventions-of-standard-english/subject-verb-agreement-and-pronoun-antecedent-agreement/v/pronoun-antecedent-agreement-syntax-khan-academy
Practice pronoun-antecent: https://www.khanacademy.org/humanities/grammar/syntax-conventions-of-standard-english/subject-verb-agreement-and-pronoun-antecedent-agreement/e/pronoun-antecedent-agreement