Linguapress Free Online English Grammar

"The Linguapress English grammar provides an overview of the main points of English grammar, clearly explained, with plenty of  examples. These pages are designed for students in the higher grades of secondary education or high school, and in universities and other forms of higher education. Though originally aimed at non-native speakers of English, this is also an invaluable resource for English-speaking students wishing to comprehend the syntax and workings of their own language".

Source and resource: http://linguapress.com/grammar/.