Apologies, the link I have attached required a password, so I will provide a written explanation. This will undoubtedly come back up before the year is out and will repeated into Year 6.
We can remember it with the following acronym:
When you see I SAW A WABUB, you are using a subordinate conjunction. They can be used in one of the following two ways:
Do you see in the second version there is a comma keeping the two clauses apart.
Have a try with the examples in the activity sheet. Complete the sentence first, then rewrite it so that the subordinate clause it at the beginning. Don't forget to add the comma in.