Structure of phrase: 當為汝說無為法，及無為道跡。
Structure of 當為汝說無為法及無為道跡.

There is only one section in this sentence as follows.
當為汝說「無為法」及「無為道跡」。

（我）應當為你們說明「無為法」及「無為道跡」。

(I) should explain to you the 無為法 and 無為道跡.
or
(I) should show you the 無為法 and 無為道跡.

無為法（Asaṃskṛta dharma; Unconditioned dharma）：
離因緣造作之法。是無貪、瞋、癡、煩惱的境界。
Anything not subject to the principle of cause and effect (因果法), nor law of dependent origination (緣起法). It is the state or realm without any greed, anger, ignorance and affliction.

無為道跡 (The path to 無為法)：
八聖道 (The eightfold noble path) or 八正道 (The eight right or correct ways)
(1) right view 正見
(2) right thought 正思惟
(3) right speech 正語
(4) right action 正業
(5) right livelihood 正命
(6) right effort 正精進
(7) right remembrance or mindfulness 正念
(8) right concentration or meditation 正定