schließen is a strong verb, so it follows this part of the tips and notes:
The Präteritum of strong verbs is not quite predictable. They usually change the stem and add -st, -en, -t, or no ending at all.
In this case, the changed stem for the past tense is schloss- (the vowel changes from long i(e) to short o) and the ending is no ending at all.
It's similar to how finden gives er fand (with changed stem and no ending).
It's the thing directly affected by the action -- the direct object. Those take the accusative case.
Calling it "receiving action" might be confusing since the recipient of giving is in the dative case (and the thing directly affected by the giving, the given thing, is in the accusative).