That part really confuses me a lot. How would I know if the relative clause needs a comma. I know extra information needs, but, when I write, I don't know if it's extra or not extra.
Companies use a wide variety of strategies to advertise goods and services, which includes billboard and newspaper.
I wish I had better English, but I work hard to improve it. I'm studying for IGCSE, so I'm only interested in BrE.
I know that "Which + present simple or present continuous" can be changed into "V+ing". But that's only when they information are not extra, isn't it?
What follows the comma is extra information.
Well, when can it be a reason?
I don't know what else to say.
No problem. Thanks a lot about your clarification.