F8
To turn on Extend mode (EXT appears in the lower-right corner of the window); pressing F8 repeatedly extends the selection to the word, the field, the record (in Datasheet view only), and all records

LEFT ARROW or RIGHT ARROW
To extend a selection to adjacent fields in the same row in Datasheet view

UP ARROW or DOWN ARROW
To extend a selection to adjacent rows in Datasheet view

SHIFT+F8
To undo the previous extension

ESC
To cancel Extend mode