The Qur'an is muSaddiq مُصَدِّق for previous scriptures. This word is an active participle form II. Active participle means that something or somebody is doing some task, in this case described by the root Sad-dal-qaf. Verbal form II is used for causing main form I verb (actions, states, happenings).
Our languages mostly don't have variety of verbal forms as the language of the Qur'an, so they cannot express all verbs correctly. For example, the root Tay-ha-ra is used for purity as state. Basic verb used as yaThurne يَطْهُرْنَ in 2:222 is talking about women in a state of pureness, but verb form II cannot mean the same, but speaks of an action (not the state) to bring something to the state of pureness, ie. to clean something. Verb form V is a reflexive of form II so it speaks of an action to bring self to the state of pureness, ie. to clean self. Even this is easy to understand for this root, it's not the case for all the roots, but anyways we should follow this pattern, that the verbal form II always describes an action by which the main meaning is caused, no matter if that meaning is an action, state or happening.
The root Sad-dal-qaf describes something correct, which can be truth, soundness, truthfulness, etc. By the Qur'an itself we can find that previous scriptures were altered in many ways, even the whole "books" were added, removed, changed. One of examples can be found in 2:102 which talks about Suleymaan wasn't kaafir, as some "books" claim, and moreover it talks about so-called "Book of Malachi" as a writ by two melek, namely Harut and Marut.
If the Qur'an claims that previous scriptures were altered, as it is, then how come muSaddiq mean "confirming"?
As an active participle form II, muSaddiq describes the Qur'an as something which persists in causing previous scriptures to be correct. If they are already correct, muSaddiq could mean that the Qur'an is giving proves for their correctness, which is not the case. All the stories in the Qur'an which can be found in previous scriptures differ, sometimes in some details but sometimes in everything.
This is why we should discard this meaning "confirming (something)" or "proving (something) to be true", and have to keep in mind that these scriptures are altered and not true. Thus, muSaddiq for the Qur'an means that the Qur'an is making them to be correct by telling what is not correct in them. In this case, this participle could be translated as "corrector".
So, the Qur'an as muSaddiq isn't confirming previous scriptures, but it is correcting them. Due to this, those who follow previous scriptures should have in mind what the Qur'an said about what they have in their scriptures and use this to correct errs in their scriptures.
