Grammar

Clause

Elementary grammar

Find-and-correct exercises

Identify-the-intruder exercises

Parts of speech

Easy Learning English Grammar
Collins Education

English Grammar reference
British Council

English grammar guide
EF

Grammar
Cambridge Dictionary

Grammar
Purdue University Online Writing Lab

Grammar
Oxford Dictionaries

Grammar Index
University of Victoria

Guide to Grammar and Writing
Capital Community College

HyperGrammar
University of Ottawa

Intermediate Grammar Reference
BBC Learning English
Plus other levels from basic to advanced

Internet Grammar of English
University College London