Let me tell you about what I did last week.
✗ Let me say you about what I did last week.
I say "Talk to you later!", but he doesn't respond.
✗ I tell "Talk to you later!", but he doesn't respond.
The verb "tell" means to communicate facts.
I tell him everyday that I love him.
The fact "I love you" is communicated to him everyday.
The verb "say" means to speak words.
I say to him everyday, "I love you".
I speak those words everyday.
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