Learn Turkish: Anladım ki bu yara öldürücü. - I understood that this wound is deadly.

It uses the suffix -(y)ıcı / -(y)ücü, which turns verbs into adjectives (or nouns) describing an agent that does the action. The root is öl- (to die). First, the causative suffix -dür is added to make öldür- (to kill). Finally, the agentive suffix -ücü is attached. Literally meaning "something that kills," öldürücü translates to "deadly" or "fatal."

At this level, I usually use -dığını for "that" clauses. Could I say bu yaranın öldürücü olduğunu anladım instead of using ki?

Yes, absolutely! Bu yaranın öldürücü olduğunu anladım is the standard, native Turkish way to embed a clause. However, using the conjunction ki to connect two separate sentences (Anladım ki...) is also very common in everyday speech. It creates a slight dramatic pause—much like saying, "I realized something: this wound is deadly." Both structures are perfectly correct to use.

Why is the English translation "is deadly" when the first verb anladım is in the past tense? Doesn't Turkish need a past tense copula here?

Unlike English, Turkish doesn't have a strict "sequence of tenses" (backshifting) after ki. The second clause simply states the fact exactly as it was realized in that moment. Since the wound is currently deadly, you just use the present tense: bu yara öldürücü (this wound is deadly). You do not need to say öldürücüydü (was deadly) unless the wound is no longer deadly.