1. Explanation | Common mistakes | Hint for next step

The base abstracts the alpha hydrogen of aldehyde to form a carbanion. The carbanion donates its electrons to form a highly resonance-stabilized enolate anion. The enolates are very strong nucleophiles.

2.Explanation | Common mistakes | Hint for next step

The enolates are very strong nucleophiles and the carbon of carbonyls is a good electrophile by nature. Hence, the nucleophilic carbon from the enolate, attacks the electrophilic carbonyl carbon, giving it a condensed product.

3.Explanation

The reaction of the alkoxide with water molecule gives the aldol condensation product between two aldehydes. The dehydration of the aldol product does not occur to form enol, since there is no alpha hydrogen that is present with respect to the carbonyl group.

please see attached file for full solution