When the asserted relation matches reality, a report has truth (ṣidq); when it does not, it has falsehood (kadhib). These are assessments of the reported content, rather than a judgment based solely on its grammatical form.
“It rained today” is true if rain occurred and false if it did not. The sentence structure remains identical in both cases.
Understanding what a report means is one matter; knowing its truth is another. A listener can understand a perfectly meaningful report while lacking evidence for it. Thus the truth-aptness of a claim and the listener's justified knowledge of that claim must be kept distinct.