If there is no danger of misreading, the word that may sometimes be omitted when it introduces a subordinate clause.
Occasionally, however, a sentence might be misread without that.
Sarah didn’t see the tree; she saw that the tree was gone. The word that tells readers to expect a clause, not just tree, as the direct object of saw.
At first, the original sentence reads as if the team discovered the site. The word that makes it clear that the team discovered what had recently happened at the site.
Exercises:
Needed words 1
Needed words 2
Needed words 3
Needed words 4