断言 and 断定: asserting firmly and reaching a definite judgment
Compare a firm declaration about completing a plan with judgments about a cause or animal type. Expressed certainty is not evidence of success or sufficient proof.
Making a firm assertion
断言 (だんげん) means asserting or declaring firmly. 「十分な証拠がないうちは成功だと断言できない。」 says success cannot be declared while sufficient evidence is lacking.
「彼女は計画を必ず完成させると断言した。」 says she declared that she would complete the plan without fail. The declaration occurred; completion is the future content of that declaration.
Making a definite judgment
断定 (だんてい) concerns a definite judgment. 「証拠が少ないため、原因を一つに断定しなかった。」 says no single cause was concluded with certainty because evidence was limited.
「この影だけでは動物の種類を断定できない。」 says the animal’s type cannot be determined from this shadow alone. この影だけ identifies the limited material available for the judgment.
Expressed certainty and its grounds
With 断言, examine a forceful assertion; with 断定, examine a definite judgment. Do not require assertions to be exclusively spoken and judgments exclusively internal.
Expressing certainty does not guarantee truth or realization. Examples that reject assertion or judgment because of insufficient evidence retain that limitation.
What action has occurred in 「彼女は計画を必ず完成させると断言した。」?
Show the answer and why
She has declared that she will complete the plan without fail. The sentence does not say the plan is complete, and 必ず does not supply evidence of completion.