Dear Rosie,
I find Purdue OWL (Grammar section) quite useful. The explanations/example sections are reasonably detailed and followed by some sample activities; worksheets are also available in printable format:
https://owl.english.purdue.edu/owl/section/1/5/ . For younger students, the British Council website might be another potential source for teaching materials and/or
activities.
I also think Clarity English provides some good quality independent study resources, for example Tense Buster
http://www.clarityenglish.com/program/tensebuster.php (can be used as individual activities in the classroom as well), but I am afraid it requires a licence.
Hope that helps to some extent!
Best wishes,
Arina
(Dr.) Arina Cirstea
Lecturer, Centre for Learning and Student Support (CLaSS)
DE MONTFORT UNIVERSITY
Leicester LE1 9BH
T: 0116 250 6351
W: dmu.ac.uk