"I kicked the ball."
Translation:Κλώτσησα την μπάλα.
I still fail to understand why τη is marked wrong here: https://screenshots.firefox.com/zm5KhaODBgmbHgVB/www.duolingo.com τη μπάλα flows more easily off the tongue than την μπάλα, which requires an change of tongue position. Can anyone explain?
I assume Greek has a rule for using τη and την;
It does indeed.
Here's how one schoolbook for Greek schoolchildren explains it:
The short version is: αυτήν, την, δεν, μην add/keep (depending on how you want to look at it) the final nu before vowels and before the plosive sounds /p t k b d g/ -- that is, before the letters or combinations π τ κ · μπ ντ γκ · ψ τσ τζ ξ.
It used to be the case that those rules also applied to αυτόν and τον in the masculine accusative singular, but the new rules say that those words always keep the final nu, so as to be distinct from neuter αυτό, το.
So for example we write είδα τη θεά "I saw the goddess" (no final nu) but είδα τον θεό "I saw the god" (with final nu) -- under the old rules, it would have been είδα το θεό.