Sometimes you need to fill in a learning gap. Sometimes you need to help a particular student with a particular skill. Online services can meet these needs if you know where to look. Online services are available 24/7 and can be accessed from any web enabled device. No one has to wait for knowledge to come to them and no teacher has to be out of resources for ever complex classes. I have picked some more sites which will help, in particular with English, but the first two cover other subject areas as well.
The sites offer online printables, online exercises and online help for classes and individuals.