I am confused

[Unregistered]

I am confused regarding the use of the word "that" when linking phrases together. Specifically, there are many cases where a sentence using the word "that" would sound just fine without the word "that".

An example:

"We have data showing THAT these systems are suitable for study"

This sentence would be fine written either way. Is one form correct over another?

A follow up question I would like help on is: what is the difference between "that" and "which"

Thank you very much for your help

[Casiopea]

"We have data showing THAT these systems are suitable for study"

This sentence would be fine written either way. Is one form correct over another?

When that introduces a clause--a subject and a verb--, it functions like a conjunctions, and so it's optional (...):

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.

A follow up question I would like help on is: what is the difference between "that" and "which".

that introduced required information:

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.