And suggest it's mode of formation from glycerol

Glycerol is a triol, meaning it has three hydroxyl ($\text{-OH}$) groups. Its chemical name is propane-1,2,3-triol.

Anhydrous magnesium sulfate ($\text{MgSO}_4$) acts as a dehydrating agent. When heated with glycerol, it removes water molecules from the glycerol structure.

The formation of acrolein ($\text{CH}_2=\text{CH}-\text{CHO}$) from glycerol involves two dehydration steps.
First, two hydroxyl groups are removed, typically from adjacent carbons, to form an enol intermediate. This enol then tautomerizes to an aldehyde.
The overall reaction involves the removal of two molecules of water.

The mode of formation involves the dehydration of glycerol, where two molecules of water are eliminated from the glycerol molecule to form the $\alpha,\beta$-unsaturated aldehyde, acrolein. This typically proceeds via an initial dehydration to form an enol, which then tautomerizes to the more stable aldehyde.