Here are some useful resources for learning Chinese Mandarin:
Basic Chinese Grammar: A Grammar and Workbook
By Rimmington & Po-Ching (Routledge) - A comprehensive grammar guide with exercises for beginners. This book provides clear explanations of Chinese grammar points with plenty of practice activities.
Chinese Grammar Index
By University of Oxford, Centre for Teaching Chinese as a Foreign Language - A list of grammatical items with examples and explanations for Chinese learners.
Chinese Measure Words 量词
By Omniglot - Introduction to Chinese measure words or classifiers with examples. Measure words are essential when counting nouns in Chinese.
Practical Chinese Reader: Online Exercises
By Xie, Tianwei - Self-tests and exercises for students who use the textbook Practical Chinese Reader. Covers lessons 1-30 with interactive practice activities.
A guide to the writing of Mandarin Chinese in romanization (www.pinyin.info)
By M. Swofford - This website is aimed at contributing to a better understanding of the Chinese languages and how romanization can be used to write languages traditionally associated with Chinese characters (such as Japanese, Korean, and especially Mandarin Chinese).