When that introduces a clause--a subject and a verb--, it functions like a conjunctions, and so it's optional (...):Originally Posted by Unregistered
The data shows (that) these systems are suitable.
'these systems are suitable' is a clause. Other examples,
EX: This is the book (that) I want.
'I want' is a clause.
EX: They rented the house (that) we used to own.
'We used to own' is a clause.
that introduced required information:A follow up question I would like help on is: what is the difference between "that" and "which".
EX: A suitcase that has no handles is useless.
If we omit the underlined portion above, the result is strange:
EX: ?A suitcase is useless.
which introduced added information--information that's not required, but added for flavour, and since the information it adds is not necessary, commas are used, like this,
EX: The UFO, which I saw, was round. ~ The UFO was round.
The above sentence expresses two things: 1) The UFO was round, and 2) by the way, I saw it.
The fact that I was the one to see the UFO is not all that important to me or to the meaning I want to express, so I added it in using 'which'. If it were important to me or to the meaning I want to express then I would use 'that',
EX: The UFO that I saw was round.
The above sentence expresses one thing: 1) I saw a round UFO.
Since 'that' in the example above introduces a clause (i.e., I saw) it's optional:
EX: The UFO (that) I saw was round.