وَلِيُمَحِّصَ (And so that may purify) is in the subjunctive mood — it comes after a particle like أن ("that") or لن ("will not"), which changes its ending.

We translate it as "to [verb]" or "that [someone] [verb]".

وَلِيُمَحِّصَ (And so that may purify) uses form II (فَعَّلَ) — the middle letter is doubled, which makes the meaning stronger or causative.

The meaning is more intense than the basic form, or means "to make someone do [action]".

وَلِيُمَحِّصَ (And so that may purify) starts with وَ, the most common Arabic word — it means "and".

We add "and" to connect this to what came before.

وَلِيُمَحِّصَ (And so that may purify) starts with لِ showing purpose — it means "so that" or "in order to".

We add "so that" or "in order to" before the verb.