"Plants are growing from the earth."
Translation:El la tero kreskas plantoj.
Strange because on the multiple choice you must select "Plantoj kreskas de la tero" if it appears.
Adding every possible correct answer would make the course tree too unweildly to maintain.
Possession is just one use of de.
A bit like "of" in English - in "the son of my mother's brother", it shows possession, while in "lots of love" or "he is afraid of the dark" or "because of the rain", it does not.
The basic difference between de and el as spatial prepositions is that el is "from out of" while de is more generally "from", or perhaps "from next to".
I think "could be" would be more appropriate, because the word order can be varied in Esperanto without changing the meaning, so "Plantoj kreskas el la tero", "El la tero kreskas plantoj", "Plantoj el la tero kreskas" and "El la tero plantoj kreskas" all mean the same thing. Of course there are limits to this freedom of word order - we couldn't for instance say, "La kreskas el tero plantoj".
Could one translate "they are to X" with estas+infinitive or is there another way to deal with that
What do you mean with "they are to X"?
Do you mean obligations, as in "They are to read this book by tomorrow and write an essay on it" ?
No, that doesn't work with "ili estas legi" or the like - you need something like "ili devas legi" (they have to read....) or "Oni ordonis al ili legi" (one has ordered them to read) or similar.
And future plans such as "They are to build a new hospital there next spring" would be "Ili konstruos novan malsanulejon tie la venontan printempon" with plain future (konstruos = will build).
Or do you mean something else?
If you wanted to specifically use the present continuous tense here (ie to unambiguously mean 'are growing' (rather than 'kreskas' that can also mean 'grow')) you could indeed use a compound tense but it'd be 'estas kreskantaj' not 'estas kreski'.
Because it's incorrect everywhere…
A better question is: Why would that be correct???
Mi volas, ke ĝi sciu la vorton „grundo“ almenaŭ por la rapidaj „testantoj“ kiel mi :)
Esperanto verbs are the same in the singular and the plural. So "My plant grows" is "Mia planto kreskas", and "My plants grow" is "Miaj plantoj kreskas". The verb is "kreskas" in both cases. The only exception to this is in compound verb forms such as "estis krekanta" ("was growing"), which if the subject were plural, would be "estis kreskantaj".
I'd say that this is one of a few very common usages of kreskigi. I even found this one in the context of kultiv-:
- tiel ke unu kultivisto kreskigas kvaroblan kvanton da greno,