必ず and 多分: certainty and probability
Compare regular patterns or firm commitments with uncertain predictions, preserving the difference between without fail and probably.
Something done without fail
出かける前に必ず鍵を確認します。 says that the speaker always checks the key before going out. 必ず, read かならず, strongly presents a regular practice or policy without exceptions.
明日、必ず返します。 is a firm promise to return it tomorrow. It conveys strong commitment but does not itself prove that the future return will happen.
A prediction with uncertainty
多分、明日は雨です。 means that it will probably rain tomorrow. 多分, read たぶん, marks the speaker's prediction rather than an established fact.
彼は多分、まだ駅にいます。 says that he is probably still at the station. It does not establish that anyone directly checked his location.
Negation and the force of a request
多分、今日は来ません。 means that the person probably will not come today. 多分 qualifies the negative prediction, not a positive claim that they will come.
必ず連絡してください。 strongly asks someone to make contact without fail. 多分連絡してください is not an equivalent request; distinguish directing an action from predicting a fact.
Does 多分、今日は来ません say that the person will definitely come today?
Show the answer and why
No. It predicts that the person probably will not come. Preserve both uncertainty and negation.