Mixed Tenses (past and present) in one sentence

[Blackwhite]

Hello,

I want to say something like this:

[Jamie (K)]

You can alternatively use the present tense in this type of context if it regards a fact that is still true or always true.

"They saw that water boils at 212 degrees." (It always boils at 212 degrees, not only at the time they saw it.)
"We discovered that this plant is edible." (It's always edible, not just at the time we discovered the fact.)

Any complete textbook or reference book on English grammar will contain this information. A book I have at hand is Grammar Dimensions 4 (4th edition) by Stephen H. Thewlis. It lists several situations in which the verb in the reported speech can remain in the present tense:

-- things that are always true
-- things that are still true
-- hypothetical statements
-- statements that were made only a very short time ago
-- future events that have not yet occurred
-- casual statements using direct quotation

[Daemon99]

Jamie,

Is there any difference between the following two statements? Are they both correct?

1) John said he'll send the documents tomorrow
2) John said he'd send the documents tomorrow
_________________
Non-native speaker of English
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I intend to live forever - so far, so good.

[Blackwhite]

Jamie,
What if I don't know whether the researchers still hold that belief to be still true?

[Jamie (K)]

[Jamie (K)]