Answer:

COULD

Answer	Crossword Clue
COULD	"How .. you!"
COULD	Was able to
COULD	"I .. eat a horse"

COULD	"The Little Engine That .."
COULD	Had the ability
COULD	"This .. be the last time": Stones lyric
COULD	"This ... be the start..."
COULD	"I'm So Lonesome I ... Cry"
COULD	Might
COULD	Was capable
COULD	"I ... Write a Book"

COULD	Would be able to
COULD	Had the know-how
COULD	"... This Be Magic"
COULD	Hypothetical word
COULD	Has the potential to
COULD	"The Little Engine That ..."
COULD	Had the ability to.
COULD	"How ... you!"

COULD	Auxiliary verb.
COULD	Was able.
COULD	"I ... a tale unfold . . . "
COULD	"I would if I ..."
COULD	"I ... a tale unfold . . . ": Hamlet
COULD	"I ... Have Danced . . . "
COULD	Was able
COULD	"I ... Write a Book," 1940 song

COULD	"I ... a tale unfold . . . ": Shak.
COULD	Manilow's "..... It Be Magic"
COULD	"What ... I do?"
COULD	"... It Be Magic" (Manilow song)
COULD	Was capable of
COULD	Possibly will
COULD	"You ... say that"
COULD	"It ... Happen to You"

COULD	"What else ... I do?"
COULD	"I ... just scream"
COULD	Had the know-how to
COULD	Had the ability to