"The child opens the apple juice."

Translation: Das Kind öffnet den Apfelsaft.

[BrianDugui]

When is it 'den' rather than das, der, die?

[quis_lib_duo]

(der) Saft = masc. noun; accusative case: (den) Saft

The juice is the direct object of the sentence, hence in accusative case.

[Sherie348622]

Will they eventually teach/explain accusative, nominative.....etc?

[ferno9107]

They explain the grammer and such in the discussion areas. I'm still trying to grasp it myself. It's one of those things that only comes with practice.

[jdrewy]

can you say den apfelsaft das kind offnet?

[Anna694674]

You might say "Den Apfelsaft öffnet das Kind" in specific contexts (verb 2nd, as Erin rightly said, but object and subject switched places). This would put the emphasis on "den Apfelsaft" and make it "this particular [bottle of] apple juice": All the bottles are opened by [whomever], but this particular bottle over there will be opened by the child.

[Erin878130]

Verb is almost always 2nd position in a sentence (Dative words send it to the end) if I remember correctly