確かに + [sentence] 確かに + [sentence] + けど / が、[sentence]
確かに means "certainly" or "it's true": the speaker confirms that something is so. It is an adverb and usually comes at the start of the sentence.
On its own, as in the last example, 確かに is a reply that agrees with what the other person said: "true", "you're right".
The commonest use is to accept one point and then add a "but" with けど, が or でも. The weight of the sentence is on the second half.
Without に, たしか確か means the opposite of sure: "if I remember right", "I think". 確かに confirms; 確か hedges.
With はず, 確かに says you were sure of something that now looks wrong.