فَسَنُيَسِّرُهُۥ (Then We will ease him) 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]".

فَسَنُيَسِّرُهُۥ (Then We will ease him) starts with فَ here marking a result — "because of what just happened, this follows".

We add "so" or "then" to show this is the result.

فَسَنُيَسِّرُهُۥ (Then We will ease him) starts with سَ, which turns a present tense verb into future tense.

We add "will" — the action hasn't happened yet.