Grammar, Usage, and Style

Grammar Essentials. The Online Writing Lab at Excelsior College provides a humorous presentation of punctuation, parts of speech, and grammatical errors.
English Grammar Guide. Education First provides a grammar overview and practice test geared for English Language Learners.
Grammar. The Purdue University Online Writing Lab (OWL) provides extensive resources for native and non-native speakers of English.
OWL Exercises. Purdue University provides online practice for sentence-level issues, including punctuation, spelling, structure, and style.)
UW-Madison Writer's Handbook The University of Wisconsin-Madison Writing Center provides a useful reference for sentence-level issues.
Writer Resources. The University of Illinois Writer's Workshop provides many useful resources, including Editing and Proofreading Strategies
Visual Explanations of Each English Tense ThoughtCo provides a useful resource for English Language Learners to understand the difference between English verb tenses.

The Excelsior College Online Writing Lab provides helpful slideshows:
- Putting It All Together--Parts of Speech (slideshow)
- Putting It All Together--Punctuation (slideshow)
Webinars: Overview Walden University Writing Center provides a variety of free webinars--live and recorded--on many topics related to writing, such as this one about basic Grammar
Grammar Videos The University of Florida Writing Program provides video presentations about grammatical concepts.

Academic Phrasebank (The University of Manchester)
The Academic Word List (Victoria University of Wellington School of Linguistics and Applied Language Studies)
As, because or since? (Cambridge Dictionary)
Collins Dictionary
Corpus of Contemporary American English (Brigham Young University)
Longman Dictionary of Contemporary English
The Oxford 3,000 (Oxford Learner's Dictionary--list of the 3,000 most important words to use in English)