Purdue OWL: Grammar
Purdue University Online Writing Lab
Purdue University's Online Writing Lab maintains free reference pages on parts of speech, subject-verb agreement, tense, articles, punctuation and sentence construction, each with worked examples and exercises. The standard first-stop grammar reference for American students.
More resources on English Grammar & Style
The Sense of Style: The Thinking Person's Guide to Writing in the 21st Century
A cognitive scientist's account of why some prose is readable and some is not: the curse of knowledge, classic style, syntactic trees, coherence. Closes with evidence-based verdicts on disputed usage rules, replacing inherited superstition with linguistic evidence.
Garner's Modern English Usage (5th Edition)
Alphabetical usage dictionary covering thousands of contested points, each judged against corpus frequency data and a five-stage Language-Change Index rather than assertion. The standard desk reference for editors, lawyers and professional writers working in American English.
Style: Lessons in Clarity and Grace (12th Edition)
A diagnostic method for revising sentences: put characters in subjects and actions in verbs, manage old-to-new information flow, then control emphasis, concision and cohesion across paragraphs. Exercises follow every principle, which is why writing programmes assign it.
A Student's Introduction to English Grammar (2nd Edition)
The one book that answers 'how does English actually work' instead of 'what should I not write'. The accessible companion to the Cambridge Grammar of the English Language, describing how English syntax actually works: word classes, clause structure, complements versus adjuncts, coordination. Descriptive rather than prescriptive, and openly critical of traditional rule folklore.
Style: Toward Clarity and Grace
Williams diagnoses why prose reads as dense and supplies repeatable revision rules built on subjects-as-characters and verbs-as-actions. Readers can rewrite tangled sentences and paragraphs into clear ones instead of guessing from vague style advice.
UQx: English Grammar and Style
University of Queensland's WRITE101x covers grammatical principles, word usage, punctuation, and sentence and paragraph structure. After finishing, you can name the parts of a sentence, correct common usage errors, and edit your own writing for clarity.