En leciono 4 aperas "Ĉu tio estas la prezidanto de Francujo? Jes, tio estas la franca prezidanto." Ŝajnas al mi ke "tio" en tio frazo estus propere "tiu." Ĉu ne?

The general rule sounds: "tio" means "that something" and "tiu" "that something (of a known set), e.g.

• Tio estas aŭto : That something is a car → That is a car.
  
• Tiu aŭto havas ruĝan koloron : That individual of the known set of cars is red → That car is red.

Now if no other set or context is specified, la tabelvortoj which end in -u are assumed to refer to people. IIUC in your example you can reason either way.

• Jes, tio estas la franca prezidento : It has not yet been established that the person is the president, so we can't use "tiu" (one of the presidents)
  
• Jes, tiu estas la franca prezidento : It has been established that we speak about a person, so we can use "tiu" (one of the persons)

There are speakers who would use tiu even in the first sentence, because it must refer to a person. Taken to the extreme tio could mean that the French president could be an alien (no political pun intended).

PS. While both Lernu and Duolingo teach only the word prezidanto, there is actually a word prezidento for a state president, leaving prezidanto to mean a chair person. I prefer to make this distinction clear.

Many thanks — tre klara. Kaj utila.

My reading was "Is that particular one of the people standing on the podium the French president?" And in that case "tiu" seemed the right word: the particular one of a group.

"Tio" seems more like "Is that a man or a bush over there in the dark?" In that case "tio" is not singling one out of a group but rather pointing out some particular thing.

Metsis:PS. While both Lernu and Duolingo teach only the word prezidanto, there is actually a word prezidento for a state president, leaving prezidanto to mean a chair person. I prefer to make this distinction clear.

Ŝainas, ke ankaŭ prezidinton ekzistas. La sama signifo kiel prezidanto.
Kaj por prezidento oni povas utiligi ankaŭ ŝtatprezidanto.

Felis'.

The tio/tiu distinction is one that I have been constantly confused by, because as the OP here points out, in some cases where the object itself is known, tio is used.

For instance, in the example given, we know the person in question is a man, and therefore one among the known group of human beings, so I would also think tiu would be correct. But apparently not, and that is not the first example like that that I've run across.

It almost seems like a crapshoot as to which one is correct.

I've generally followed this rule of thumb:
tio = that
tiu = that one

It's inexact, but that's that the thought in my mind. "Kio estas tio?" would be used about some unrecognized apparatus, and "Kio estas tiu?" would be used about one of several unrecognized apparatuses. The meanings (for me) are, respectively, "What is that?" (the first example) and "What is that one?" (he second).

Leisureguy:I've generally followed this rule of thumb:
tio = that
tiu = that one

That is a good rule of thumb.

Koro_Kokoro:
Ŝainas, ke ankaŭ prezidinton ekzistas. La sama signifo kiel prezidanto.
Kaj por prezidento oni povas utiligi ankaŭ ŝtatprezidanto.

There is a verb prezidi, to lead the speech, to act as a chair person. From that verb one can create substantivajn participojn in the normal way.
 

• prezidinto : persono kiu prezidis, a person who earlier/formerly acted as a chair person
  
• prezidanto : persono kiu prezidas, a person who acts as a chair person
  
• prezidonto : persono kiu prezidos, a person who is going to act as a chair person

In practice only the -anto form is used.

Although among the definitions in PIV for prezidanto is republikestro (and ŝtatprezidanto would also do), I still prefer prezidento for republikestro, because I fail to see what leading a republic has to do with acting as a chair person. Cf. The parliament has a prezidanto. But you may prefer otherwise.

Leisureguy:I've generally followed this rule of thumb:
tio = that
tiu = that one

Tute male.
tio = that thing
tiu = that
kio = what
kiu = which; (sen sekvanta substantivo) = who

Mi pensas tio estas io, kio oni lernas pli bone per praktiki (legi, legi, legi kaj konscie percepti, reprodukti, legi, legi, ...) ol per tro pripensi.

I think this is something one learns better by practicing (reading, reading, reading and consciously perceiving, reproducing, reading, reading, ...) than by thinking too much.

Metsis:

Koro_Kokoro:
Ŝainas, ke ankaŭ prezidinton ekzistas. La sama signifo kiel prezidanto.
Kaj por prezidento oni povas utiligi ankaŭ ŝtatprezidanto.

There is a verb prezidi, to lead the speech, to act as a chair person. From that verb one can create substantivajn participojn in the normal way.
 

• prezidinto : persono kiu prezidis, a person who earlier/formerly acted as a chair person
  
• prezidanto : persono kiu prezidas, a person who acts as a chair person
  
• prezidonto : persono kiu prezidos, a person who is going to act as a chair person

In practice only the -anto form is used.

Although among the definitions in PIV for prezidanto is republikestro (and ŝtatprezidanto would also do), I still prefer prezidento for republikestro, because I fail to see what leading a republic has to do with acting as a chair person. Cf. The parliament has a prezidanto. But you may prefer otherwise.

Dankon por ĉi tiu via poŝto. Kion oni povas igi ĉio specife kon participoj, tion mi devas rigardi eĉ pri detale. Sed ĝustnun mi estas en la kansajo dialekto..

Ĉion bonan.

Felis'.