"Tiu fadeno estas unu metron longa."

Translation: That thread is one meter long.

[Las_Excadenas]

Why is 'metron' in the accusative?

[Komponisto]

The accusative is used when talking about distance or length. https://www.duolingo.com/skill/eo/Numbers

[Siavel]

Then why is "longa" not in the accusative. The link demonstrates this, but doesn't explain why.

[tommylinsley]

Like other sentences that contain "estas", there is no accusative direct object "-n". "Metron" takes the -n simply because -n is used with measurement. "-N" is used for a few other purposes that are not related to Direct Objects. Measurement is one of those other purposes.
According to Bertilo Wennergren (member of Academy of Esperanto): "The typical role of the accusative is to mark the direct object, but accusative has other uses too. That's standard terminology. "Accusative" is the name of a case. Cases have various uses.

"Somehow the idea that only N for direct object can be called "accusative" has spread a lot. It's however not correct."
https://www.facebook.com/groups/duolingo.esperanto.learners/permalink/564207133741308/?comment_id=564211870407501&comment_tracking=%7B%22tn%22%3A%22R2%22%7D

[tommylinsley]

I forgot to add that, personally, I have never seen a logical explanation as to exactly why a unit of measure would need to marked with an "-n". I do understand the logic of marking direct objects, and surely there must be some logic explaining the marking of measurements.

[jungerstein]

Perhaps it stems from Latin usage; see https://la.wikipedia.org/wiki/Accusativus#Accusativus_mensurae

However, it needs further evidence to state whether this usage of accusative in Esperanto was herited from Latin.

[matt.burns]

Thanks!

[GnagerPrinsen]

Same in German (der Mann ist einen Meter gross), never understood why.

[luizjoaquim]

Could I say "Tiu fadeno havas unu metron da longeco"?

[Alogician]

It would make sense... so why not! (Except, "longo" means length - "longeco" roughly meaning "the quality of being long" so is for remarking rather than measuring.)

[Javi-Er]

The voice is saying "unu metrono longa" or is it just me?

[McAllFather413]

I put in "That thread is one meter" snd Duo is like "Nu. You forgot the long."

[Gaelgeoir]

Well, I'm glad I'm not the only one fretting about this. I only knew of two uses for the -n
1. direct object and 2. to show movement towards.
I have also come across "Mi venos lundon" - "I shall come (on) Monday"