De. Da.
"De" means "of", "from", or "by"; it denotes (i.) "possession", as "La
cxapelo de la patro", The father's hat; (ii.), "the agent of an action",
as "La letero estis skribata de Johano", The letter was written by John;
(iii.), "cause, Li mortis de febro", He died of fever; (iv.), "starting
point, from, since", as "De tiu tempo neniu lin vidis", (Starting) from
(since) that time nobody has seen him; "Li venis de Parizo", He came
from Paris.
"Da" means "of"; it is used after words signifying quantity, when the
noun following has a "general", not a "particular" sense, as "Dekduo
da kuleroj", A dozen spoons; "Funto da teo", A pound of tea; "Peco da
pano", A piece of bread; but if some particular tea, bread, etc., is
specified, "de" must be used, as "Funto de la teo", A pound of the tea;
"Peco de la pano", A piece of the bread. "La" cannot be used after "da."