If a word ends in a single consonant preceded by a single vowel, and the stress falls on the last syllable, then the consonant is doubled before adding a suffix that begins with a vowel.
Let's break it down:
* Single consonant: The word "grab" ends in the single consonant "b".
* Preceded by a single vowel: The "b" is preceded by the single vowel "a".
* Stress falls on the last syllable: The stress is on the last syllable "grab".
Since all these conditions are met, the "b" is doubled when adding "-ed" to form "grabbed".
This rule helps maintain the pronunciation of the word. If we didn't double the consonant, the "b" would be pronounced more lightly, changing the sound of the word.