There is a difference between being an ESL teacher and a native speaker of English, or an English (as a first language) teacher, or a professor in a university composition program, or even a highly proficient user of English as a second or additional language. In order to understand, correct, explain, and teach language (grammar, vocabulary, discourse) to English-language learners, you need to approach the structure of English from a pedagogical perspective, including learning the metalanguage (jargon) of the field.
The following books and resources might be of interest to teachers who need to brush up on their pedagogical grammar (in no particular order). Amazon links are provided, but teachers should contact publishers directly if adopting the books.
- Celce-Murcia & Larsen-Freeman, The Grammar Book: An ESL/EFL Teacher’s Course (3rd ed), Cengage, 2015. [new edition of a massively popular and comprehensive textbook. Also massive, simply in terms of weight. A good balance of structural and functional descriptions of language. Can be a little daunting, but this is still my go-to when I can’t answer a question.]
- Folse, Keys to Teaching Grammar to English Language Learners. (Michigan, 2009). [Clear and elegant descriptions of just the grammar that teachers absolutely need to know. Less technical than other books, but with more teaching suggestions. I’m using this to train K-12 teachers.]
- Cowan, The Teacher’s Grammar of English (Cambridge, 2008). [We like this book because it has a similar to scope to The Grammar Book but is more friendly to novice teachers.]
- Biber et al., The Longman Grammar of Spoken and Written English (Longman, 1999). [A massive and authoritative corpus-based descriptive grammar – a book you want to get your department to buy for the library so you can refer to it!]
- de Oliveira & Schleppegrell, Focus on Grammar and Meaning (Oxford, 2015). [Very accessible introduction to teaching ESL grammar from the perspective of Systemic Functional Grammar. This book introduces functional metalanguage and shows its application primarily for K-12 contexts, but we’ve found it very helpful in higher education.]
- Hinkel, Teaching Academic ESL Writing: Practical Techniques in Vocabulary and Grammar (Erlbaum, 2003) [excellent research-based advice for teaching language skills to advanced students in writing classes]
- Ferris, Treatment of Error in Second Language Student Writing (2nd ed). (Michigan, 2011) [not a grammar book, but a clear and comprehensive guiding to giving effective corrective feedback on ESL writing]
The following textbooks are designed to be used in class or for self-study, but they are useful for teachers looking for ways to explain grammar, especially for very advanced learners in university courses (in no particular order, other than shamelessly listing my book first!)
- Caplan, Grammar Choices for Graduate and Professional Writers (Michigan, 2012). [Designed for use in pre-matriculation and graduate-support programs, or for self study by L1 or L2 graduate students looking to improve the effectiveness of their language use, but may also be useful for teachers looking for ways to explain clause structure, clause combination, hedging, boosting, and cohesion — old/new information, paragraph structure, etc. Also includes a chapter on using corpora for language learning.]
- Frodesen & Wald, Exploring Options in Academic Writing: Effective Vocabulary and Grammar Use (Michigan, 2016). [Corpus-based approach to choosing vocabulary and grammar resources in order to effectively express common types of meanings and relationships in academic writing, especially undergraduate level.]
- Ferris, Language Power: Tutorials for Writers (Bedford/St Martin, 2014). [Aimed specifically at first-year composition – or similar – courses, this is a nice book for a supplement, tutorial program, or self-study, with excellent advice for all writers, but especially L2, crossing between language use and rhetoric.]
- Conrad & Biber, Real Grammar: A Corpus-Based Approach to English (Pearson, 2009). [An eclectic but interesting selection of units derived from research into the Longman corpus. More useful in my experience for inspiration than as a class text.]
Online resources: I’m a little nervous to recommend websites because there are few that are well enough edited to be actually useful. Grammar seems to inspire a great deal of misinformation. However, these resources are reliable:
- Purdue Online Writing Lab – resources for ESL teachers and writers
- University of North Carolina Writing Center — handouts for everyone — ESL resources
Please feel free to suggest additional resources in the comments below!
Nigel Caplan 
I would recommend adding the “Elements of Success” series by Anne M. Ediger, Linda Lee, Randee Falk, and Mari Vargo. The language in all activities and exercises is entirely corpus-based. A very different approach to teaching and learning grammar. (Disclosure: I am a former student of Professor Ediger’s.)
https://elt.oup.com/catalogue/items/global/grammar_vocabulary/elements_of_success/?cc=global&selLanguage=en&mode=hub
From the Oxford ELT website:
“A refreshing new grammar course that presents grammar clearly, simply, and completely. …..Elements of Success is a new grammar course which helps students learn the real-world grammar they need to read, communicate, and write effectively. Grammar is clearly presented with highly visual charts and explanations, and extensive four-skills practice helps students apply and remember what they learn.”