"Eu vorbesc celui de-al doilea copil."
Translation:I speak to the second child.
Hum... I couldn't really get the expression "celui de-al doilea". What is the meaning and the category of each word in this sentence? And how is the dative expressed here?
This construction is a bit tricky. In general, "cel" is the so-called "demonstrative article," "the." In this case, however, it is used idiomatically as a particle in the formation of ordinal numerals in the various cases.
Ordinal numeral = "cel de- / cea de- / cei de- / cele de-" + ordinal numeral. They mean the same thing, it is just a matter of emphasis.
Consider: "E al doilea". ("He / it is the second"; nominative case). The same can be expressed a bit more emphatically as: "E cel de-al doilea" ("He is the second one / that second one"; nominative case; literally this is like saying "that of the second one"). The feminine form is "cea de-a doua". One way of putting it is that the latter is a case of greater individualization.
Now, the second form in the dative case become "celui de-al doilea" (to the second one). The feminine singular equivalent is "celei de-a doua".
Another example: "Al treilea concurent a evoluat mai bine decât al doilea". = "Cel de-al treilea concurent a evoluat mai bine decât cel de-al doilea". (The third competitor performed better than the second one. )
Note that the ordinal numerals "primul / prima", "secund / secundă", and "terţ / terţă" cannot be preceded by "cel de- / cea de-".
I hope this helps!