I'm not really sure what you're asking, so apologies if I'm saying things you already know, but basically there are three differences between these verbs a) use in direct and reported speech b) syntactic patterns and b) collocational relationships :
a) Direct/Reported speech Say can be used in both -
She said "I can't do it" or
She said that she couldn't do it.
However,
tell can
only be used for reported speech :
She told me that she couldn't do it b) Syntactic patterns
As you indicated, the verbs are used with different syntactic patterns :
- tell
somebody something :
She told me that she couldn't do it
- say something :
She said she couldn't do it
There is the "to" option, but this is rare - I checked in a
Concordancer and in the first 40 examples of
said, it didn't occur at all. If we want to specify the person, then the natural choice is to use
tell.
So, in all the examples so far,
say and
tell are interchangeable unless we specifically want to specify who was being spoken to - in which
tell would be the natural choice - in other words we'd be more likely to say :
I didn't tell John, but I told Mary than
I didn't say it to John but I said it to Mary - the latter is grammatically correct but sounds a bit unnatural.
c) Collocational Relationships
Where these two do differ is in their use in set phrases. In the following phrases they are
not interchangeable :
say a prayer /your prayers
say the alphabet
tell a story
tell the time
tell a lie / the truth
and various other set expressions which you can find in a dictionary if you want to.
tell fortunes
two verbs 
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