Whenever my father takes an umbrella out, he always leaves it somewhere.

Literal

Father [topic-は] umbrella [object-を] carrying goes-out [then-と], always somewhere [to-へ] leaving-behind come.

持って出かけると (whenever he takes it out) + 必ず (always) + 置き忘れてくる (leaves behind and comes back). 置き忘れる is a compound verb (置く + 忘れる = leave behind by forgetting). A vivid character detail.