What Catholic teaching did Dei Verbum emphasize?

Answer 1

Dei Verbum (Dogmatic Constitution on Divine Revelation), which you may read at the link below, taught on the revelation of God which is contained in Sacred Tradition and in Sacred Scripture. It also covered Biblical inerrancy and infallibility, extracted from the Wikipedia article on Dei Verbum:

Concerning sacred Tradition and sacred ScriptureIn Chapter II under the heading "Handing On Divine Revelation" the Constitution states among other points:9. Hence there exists a close connection and communication between sacred Tradition and sacred Scripture. For both of them, flowing from the same divine wellspring, in a certain way merge into a unity and tend toward the same end. For Sacred Scripture is the word of God inasmuch as it is consigned to writing under the inspiration of the divine Spirit, while sacred tradition takes the word of God entrusted by Christ the Lord and the Holy Spirit to the Apostles, and hands it on to their successors in its full purity, so that led by the light of the Spirit of truth, they may in proclaiming it preserve this word of God faithfully, explain it, and make it more widely known. Consequently it is not from Sacred Scripture alone that the Church draws her certainty about everything which has been revealed. Therefore both sacred tradition and Sacred Scripture are to be accepted and venerated with the same sense of loyalty and reverence. Footnote: Cf. Council of Trent, Session IV, loc. cit.: Denz. 783 (1501).