I wrote "This is one liter", and it got accepted as correct. *30.12.2015
"Litre" and "liter" are both accepted.
I'm not used to the American spelling (of a metric word!), so every time I see it, it looks like "lighter" to me, not "leeter".
We are still in nominative case because the subject is being described.
The verb "sein" (= to be) is special, both the "subject" and the "object" take the nominative case. It's sort of like how in older English they said "I am he" rather than "I am him".
So while "I have a liter" = "Ich habe einen Liter"
"This is a liter" = "Das ist ein Liter"
Can someone tell me when to use 'das' and when to use 'diese' for 'this'. The question is probably out of context here.. but I can never understand it.
In colloquial German "dies" is definitely less common. So, if you use "dies" instead of "das" it sounds more nerdy or posh.
Other than that., if you use "dies" you'd probably be more likely to be pointing at the object than if you used "das", for instance, on a map or on a picture. "Dies ist mein Auto." - "This is my car" (pointing at a picture of it). But it's more common to say "Das ist mein Auto." whether it'd be in a picture or in real life.
Also, you use "dies" (or declinations of it) if you previously mentioned the object you're talking about.
For instance, say that with your first car you drove all around Europe. You'd introduce it with "Mein erstes Auto war ein Käfer" - "My first car was a beetle."
"Mit diesem Auto bin ich nach der Schule durch ganz Europa gefahren." - "With this car I drove through all of Europe right out of school."
For the second one, it is in the accusative case. 'Er' is the subject and 'Liter' is the direct object, therefore 'ein' becomes 'einen'. You can refer to this website if you are confused about how German cases work - http://www.jabbalab.com/blog/795/how-the-german-cases-work-nominative-accusative-dative-and-genitive
The male voice is very weak on the long vowels, like here. He says litter, not Liter.