Omitting "that"

It depends on the grammatical function.

1. When it's a conjunction - ie in sentences like :

They said that they couldn't do it./ They said they couldn't do it
It's possible that he doesn't know about it. /It's possible he doesn't know about it.

then there is no "rule" - it's a matter of speaker choice. Generally, it's omitted in everyday spoken language, but is more likely to be included in more formal varieties of the language.

2. When it's a demonstrative adjective (Give me that book) or pronoun (I saw that!) it's obligatory.

3. When it functions as a defining relative pronoun, it follows the ordinary rules for relative pronouns - ie it can be used or omitted when in object position :

I saw the man that/who she talked to.
or I saw the man she talked to.

but is obligatory in subject position:

I need the train that/which goes to Oxford.

Hello,
I have problems with "that" when it acts like subordinate conjunction. Are there some "rules" (that?) we can refer to when we want to omit it? I know it can be deleted after "frequent" words such as: know, say, find, feel, seem(?), believe (refference Grammar Girl : Omitting “That” :: Quick and Dirty Tips ™ )
It seems (that?) it helps if we learn those words which permit omission by heart.

Thanks

As I said - it can always be omitted when it's a subordinating conjunction, and in everyday conversation usually is. It makes no difference what it follows, so there's no point learning a list. If you're doing anything other than eg giving a speech, writing an essay etc, omit it. It's a matter of speaker/writer choice, determined by the level of formality that you want to achieve.

Thanks.