Some Idioms

fallen off,  To drop or come down freely under the influence of gravity: Leaves fell from the tree.

fall in with : agree to
Rob was happy to fall in with her plans


 fallen through

[for something, such as plans] to fail. Our party for next Saturday fell through. I hope our plans don't fall through.

fall back

move or turn back; retreat.

"the enemy fell back into a defensive position"

fall apart

break up, come apart, or disintegrate.

"their marriage is likely to fall apart" 

fall behind, .

fail to keep up with one's competitors.

"Britain has fallen behind in the space business"

fall into line conform with others.

"she defiantly pledges not to fall into line with the masses"