Here’s an example with an aldehyde.

Addition of a nucleophile to a carbonyl will give a tetrahedral intermediate, and predicting what happens next is the same as predicting the carbonyl’s chemistry. The reaction may reverse, stop there, or collapse to a new carbonyl compound (which may or may not go on to react again).

Addition to an aldehyde gives a tetrahedral intermediate:

What happens to the tetrahedral intermediate next? In the general scheme there were four possible outcomes. With aldehydes there’s only one pathway:

The reaction stops after addition of one nucleophile:

Grignard addition to aldehydes always gives secondary alcohols.

Grignard addition to ketones always gives tertiary alcohols.