。
You can't drill new tricks into an old dog.
Literal
Aged dog [to-に] new trick [object-を] drill-in [nominalizer-こと] [topic-は] can't.
Another phrasing of the same proverb. 教え込む ('to drum into, drill thoroughly') is a compound verb where the ~込む suffix adds the nuance of thoroughness or pushing something fully in. 芸当 (げいとう) is slightly more formal than plain 芸 ('trick, art').