Run on vs not run on [duplicate]

In general, when can I omit that from a sentence?

Can I omit that (emphasized) in sentence (2) below?

1. We say that such algorithms handle concept drift and can learn from time-changing data streams.
2. When the statistical properties of the target variable change, we say that concept drift has occurred.
3. She told me that some of her classmates failed.

To me, sentences (1) and (3) sound better without that. If I omit that from (2), it just doesn't sound right when I read it out loud. (I should point out I'm not a native speaker.)

How do you decide when to omit it? Is it just a matter of taste? (I know that is sometimes mandatory.)

Can you think of sentences that "sound better" with that? Can you think of sentences that "sound better" without that? Can you think of a few sentences where it is wrong to omit that?

All three of your examples use a "that" which is a marker of clausal subordination: in your case, that "that" marks the beginning of a declarative content clause.

Sometimes that "that" marker is obligatory, sometimes optional, sometimes not allowed. There are some related rules on this, but there is no one simple general rule. It's something that native English speakers just pick up while speaking and listening and reading.

Note: Your "that" is NOT a relative word or relative pronoun. (There is another "that" which is the marker of clausal subordination for that-relative clauses. Some grammars consider that the two markers to be, or could be considered to be, the same. In any case, the "that" marker for relative clauses has a different set of rules in regard to its presence or absence than the one marking declarative content clauses.)

One rather firm rule--well, somewhat a firm "rule"--is that if the declarative content clause is the subject of the main clause, then the "that" marker is obligatory. That is so the reader will get a heads-up to realize that the content clause's subject is NOT the subject of the main clause, even though it is located at the beginning of the sentence.

There are a whole bunch of more similar rules, but they are all rather specific as to the syntactic situation that they are talking about. If the rules were simplified or made too general, then there would be too many so-called exceptions. Native English speakers know these rules implicitly (their ear does all the work), but they would usually be hard pressed to explain them--and if they attempted to explain them, their explanations would often be wrong and/or misleading.