When “that” is necessary

"That" has several uses, but in two of them, it is possible to omit it:

1. When it is a conjunction introducing a "that" clause which is not the subject, as in your example. But when a "that" clause is used as the subject (quite rare), "that" cannot be omitted - "That she left him wasn't very surprising". But we usually prefer to use a construction with the "dummy" subject "it" - "It's not surprising (that) she left him." But sometimes the sentence flows better with "that" left in.

2. When it is a relative pronoun in a restrictive (defining) relative clause which does not refer to the subject. - "This is the hotel (that) I was talking about."

http://www-personal.umich.edu/~jlawler/aue/that.html
http://www.dailywritingtips.com/that-is-not-always-necessary/

Ah. So, basically, "that" can be omitted if it's omission leaves us with a sentence that sounds grammatically correct (at least to us native speakers).

So in my above example both phrases are correct?

Wow. That cleared things up quick. I typically prefer not to use "that" when it sounds fine without it, but I wasn't sure if it only sounded proper because we often omit it when we speak.

Thanks!

Yes, both your examples were definitely correct. And I'm sure that if it sounds grammatically correct to you as an educated native speaker, it is because it is grammatically correct (or rather vice-versa - grammatical "correctness" can be judged by what sounds appropriate to an educated native speaker).

Where I have a problem is with a sentence like -"So this means that when he leaves there will be a vacancy" - I'm pretty sure you can leave "that" out here, but it sounds better left in to me. On the other hand - "So this means there will be a vacancy when he leaves" (without "that") sounds fine to me.

I think it must be something to do with putting an adverbial phrase ("when he leaves") between "that" and the subject ("there"), but I haven't come across anything about it to say (that) "that" is compulsory here. Perhaps somebody else can shed some light on that one.

"That is the hotel about which I was speaking"

Yup. Demonstrative pronouns (and determiners - "that book you're reading") can't be omitted.

I totally agree, Warsaw. Your first example sentence sounds better--more articulate.

@chancery.co - Yes, no question about that one. However, as Warsaw points out, the "that" in your example is a demonstrative pronoun; do you know the syntactic category to which the other "that" belongs? This is what I'm actually curious about. If I knew that, I may be able to learn a little more about this "optional" item.

"That" can be a demonstrative pronoun or a relative pronoun. When "that" acts as a relative pronoun, it is usually a part of a noun clause or a restrictive adjective clause. Finally there are cases where "that" can be used as the head of a subordinate adverb clause.

That in a noun clause:

"He said that I was wrong."

And:

"That one can see is a blessing." (Odd word structure in my opinion but can be effective)

As an adjective clause

"My dog that ate the cat's food ran at my neighbor."

Finally, it's odd to see "that" used in an adverb clause and I don't have a good example to give.

@jayles, when "that" starts a noun clause (i.e. a "that" clause), as in your first example, I would suggest (that) it's a conjunction, not a relative pronoun. It's only a relative pronoun when it introduces a relative clause (what some would call an adjective clause).

Basically, we can only omit "that" when it starts a clause. As far as I can see, that means nearly always with a that clause (noun clause), unless, as in jayles' second example, the "that" clause is the subject. Oxford Advanced Learner's Dictionary puts
"that" in brackets in all its examples of "that" clauses where it is not the subject.

And it means under certain conditions with a relative clause (as outlined in my earlier comment). But "that" can't be omitted in jayles' example about the dog, because it refers to the subject of the verb in the relative clause ("ate"). On the other hand, in "This is the neighbour (that) my dog ran at." - "that" refers not to the subject, but to the object of the preposition "at", so can be omitted.

I don't think "that" can be used as a conjunction to introduce an adverb clause,
but it can certainly be used as an adverb, modifying an adjective - "Come on! The film wasn't that bad", or another adverb "It can't be that far now", and in British idiomatic use to mean something like "so" - "I was that knackered (exhausted), I couldn't go another yard", "We were that close to winning".

So we have 5 uses of that, in only two of which can it be omitted.

Conjunction in "That" clauses - "She said (that) she would be late."
Relative pronoun in restrictive / defining relative clauses - "This is the house (that) I was talking about."
Demonstrative pronoun - "That's just what I said."
Demonstrative determiner - "That meal was delicious."
Adverb - as above

@jayles - By the way, although I have withdrawn from the fray, I still keep an eye on the Anglish page, and your Newcastle song would be perfectly understandable to any Scot. "Haud yer whisht" is very common for "be quiet", even among speakers of standard Scottish English.

@Warsaw Will,

I am not Jayles. And on the note of that not being an adverbial subordinator, my Warriner's grammar book says that subordinators of purpose are "that, so that, and one other that slips my mind". I have seen and used "so that" to mark purpose but have never seen that (and honestly, I was hoping you might have seen the usage of "that" as subordinator). However, in this case, my Warriner's may be wrong because whenever I search for "that" as subordinator, I find nothing.

@Jasper - Firstly, humble apologies for misidentifying you. I'd been having a bit of a discussion with jayles recently and got confused.

Secondly, to the nitty gritty. I have no problem with "so that" as a subordinator / subordinating conjunction (and you could also have "in order that"), but I couldn't think how "that" would be used alone in this meaning. (But see further on).

Googling "subordinator of purpose" mostly seems to only bring up "so that" . Most (of the not very many) sites I've looked at don't seem to list "that" as a subordinator, YourDictionary, however, does (but without an example, unfortunately). Furthermore, an academic paper I came across suggested (that) "that" was used this way in Old English.

There are also other constructions with "so ... that", as in "She was so tired (that) she went to bed" - does that count as a subordinator? I think it probably does, even though it comes at the end of the subordinate clause rather than at the beginning. It expresses reason rather than purpose, of course. And it appears that this is another example where we can omit "that".

Talking of which, (and going back to the original question) Oxford Advanced Learner's Dictionary suggests that we are more likely to omit that in that (noun) clauses when it
follows a reporting verb or adjective, but less likely to do so when it follows a noun, as in "The fact (that) he's older than me is not relevant."

https://www.dlsweb.rmit.edu.au/lsu/content/4_writingskills/writing_tuts/linking_LL/subordinators.html

OK, Jasper, I've found some, with obligatory (apparently) may/might:

"We eat that we may live."
"He ate that he might not die."

http://www.englishpractice.com/improve/sequence-tenses/

But these don't sound very natural to me, and when I Google "that he may", in this sense, I get mainly passages from the Bible - "For I have chosen him, that he may command his children and his household after ..." - But I think it sounds rather old-fahioned. There are several examples in one paragraph in this translation of the eighteenth century "The System of Nature":

http://books.google.pl/books?id=h50d39hfIAoC&pg=PA11&dq=%22that+he+may%22&hl=en&sa=X&ei=MuF8UZiYFI3EPKLFgeAI&redir_esc=y#v=onepage&q=%22that%20he%20may%22&f=false

I personally use "that" in written communication and omit it in spoken communication.

The reason is that if I had left out "that" in the beginning of this sentence, it is less fluent. Rewritten, I could say, "Leaving out the word 'that' at the beginning of this sentence is a good illustration of my point: it's less fluent," but since I so often structure my sentences with "he said," "the reason is," "I thought," and whatever other "that"-needing subject first instead of the opposite way, I use the word "that" to make it sound more fluid.

I omit it in spoke communication, because the inflection of my voice and rhythm of the sentence leaves a lot more room for implied words.

In this sense (as a "complementizer"), "that" is optional. Using it can make more complex sentences clearer. It can also help make a sentence more formal.

In spoken and informal written English, omit as much as you want, as long as it sounds clear.
In formal written English, avoid any omission.