Question

Good morning!
1- How is it possible to formulate, for example, scientific theories? After all, even if an argument has a truth value of true or false, that is only when the speaker asserts a fact whose validity the listener can verify. But here it is currently impossible to verify it (especially since sometimes a theory is formulated through deduction, etc.—Bergson). If so, apparently this is just a nonsense sentence?
Put differently: how can a statement have a truth or falsity value retroactively? At the time it was said (for example: “Tomorrow it will rain”), it could not be confirmed, so isn’t it nonsense?
2- Can there be a sentence that states a fact (that is, an argument) when a person says it to himself, or is that merely an error, or neither false nor true (for example: if a person convinces himself that the concept “table” is “chair,” did he make a mistake or did he lie)?
Thank you very much!