"That land and house belong to our family."
Translation:Tiuj tereno kaj domo apartenas al nia familio.
Yes - if an Esperanto adjective modifies two (or more) things then it is plural.
"La bluaj pordo kaj fenestro" = the blue door and window.
And "tiu" works like adjectives grammatically.
Conversely, if you have a blue book and a red book, you can say "la blua kaj ruĝa libroj" -- here the adjectives are singular (because there's only one of each colour) while the noun is plural (because there are two books in total).
Esperanto works differently here from English.
So I guess both approaches are logical, but in English it's "this door and [this] window", while Esperanto expresses this as "these (door and window)".
Is this common in other languages?
I'm not sure whether I've come across it in any other language so far...
This surprises me as well. In a way it makes sense, but it's not very common. I've encountered probably the same issue in the Danish course with a sentence The flag's colours are red and white. The accepted Danish translation used røde and hvide, i.e. plural forms for red and white. What would be correct Esperanto translation? Personally, I'd turn the colours into nouns: La koloroj de la flago estas ruĝo kaj blanko. How stupid is that?
As a dane I can say that using "røde" and "hvide" in this case is wrong, as it implies that there are more than one shade of red and/or more than one shade of white. Well, technically you can't have a shade of white, but when the white color is so dominant that it can be hard to impossible to tell the exact hue of the color, we group them under the white colours.
So saying "flagets farver er røde og hvide" is actually "the flag's colours are within the groups of red and white colors and one or both of the groups are represented by more than one color"
You can also be more specific and only add -e to one of the colors e.g. "flagets farver er røde og hvid" which would mean that there are two or more of the red colors and the specific color "white".
But in reality there is only one red color(though not pure red) an one white color(pure white), wherefore the only correct translation would be "flagets farver er rød og hvid"
And "røde" and hvide" are not only the plural forms but also the definte forms e.g. "A red house" -> "Et rød(t) hus", "The red house" -> "Det røde hus". But when name of the color ends in a wovel we do not never add -e. E.g. "blue" -> "blå"; "A blue house" -> "Et blå(t) hus"; The blue house" -> "Det blå hus" and due to this you would encounter ambiquity if the colors in the sentence were blue and gray instead of red and white (though there are ways to avoid ambiquity)
And please note the (t) is never added unless the noun is singular , indefinite and neuter, in which case it is optional though recommended, and adding -t or -e (where possible) effectively turns the colors into adjectives.
Speaking grammatically, "tio" is a demonstrative pronoun while "tiu" is a demonstrative determiner.
What that means is that "tio" stands by itself, instead of a noun. I doesn't have a plural form.
"Tiu", on the other hand, stands before and together with a noun, and has a plural if what it stands before is plural (either many of the same thing as in "tiuj libroj" = those books, or multiple single things as in this sentence, "tiuj tereno kaj domo" = that land and house).
Sometimes, the noun is not stated, but then there is always a particular noun implied. In English, we usually can't do thisi and have to add a dummy noun "one", giving "that one".
For example, if you know that you are talking about dresses, you might say "This one is too big and that one is too small" (Ĉi tiu estas tro granda kaj tiu estas tro malgranda). You're identifying particular dresses out of a group of dresses.
"Tio", on the other hand, is used when you don't have a particular noun in mind and are simply pointing to something or introducing a new topic into the discussion. Since it's new, you "don't know" whether it's plural or not :) which explains why it has no plural form.
German, which I see you know moderately well, works similarly: "Das sind Blumen", using "das" (neuter singular!) together with "Blumen" (feminine plural) -- corresponding to Esperanto "Tio estas floroj".
But "Die Blumen da sind rot und die da sind gelb" (Tiuj floroj estas ruĝaj kaj tiuj estas flavaj; Those flowers are red and those [ones] are yellow), with plural "die" when you know that you are talking about flowers.