I can't seem to work out the difference between da and de on Duolingo (admittedly I haven't spent long on it). Could someone give an exact distinction?
The notes in the Duolingo lesson you refer to
are actually very useful, they start off
Although both da and de can be translated into English as of, they have different meanings:
Use da when you're talking about quantity.
Use de when talking about possession
taso da teo - a cup of tea (quantity)
la patrino de la knabo - the mother of the boy /the boy's mother
It goes on to give lots of examples and ends with the subtle case:
When you're talking about a quantity of a specific thing or set of things, >as opposed to a type of thing, you use de la.
Mi bezonas 5 kilogramojn da sukero - I need 5 kilograms of sugar.
Mi bezonas 5 kilogramojn de la sukero - I need 5 kilograms of the sugar.
Beyond the Duolingo notes, here is an easy trick for English speakers: if you can replace of with made of or made up of in the sense of measure/quantity, use da.
One kilogram of feathers - One kilogram made of feathers - kilogramo DA plumoj
One cup of tea - One cup made of tea (cup is a measure unit here) - Taso DA teo*.
Note: for made of in the sense of material, use el.
One cup made of clay - Taso EL argilo.