'to fall apart' means to break into pieces because it is badly made.
- It was a cheap dress. The first time I wore it, it fell apart.
- I had terrible problems assembling the shelves, then they fell apart as soon as I put something on them.
'to fall apart' also means to stop working properly or efficiently.
- The group fell apart when two or three members left and no one replaced them.
- Without Joe's leadership, the department just fell apart and was unable to work as it had before.
'to fall back on something' means to use something when everything else has failed, to use something reliable.
- If I don't get this job, I don't have any savings to fall back on.
- They don't have very much capital to fall back on if this venture fails.
'to fall behind' means to do something more slowly than others so that you are behind.
- I couldn't walk as fast as everyone else and soon fell behind.
- When she fell behind the other runners, no one thought she would catch up and win the race.
'to fall behind' also means to not be on schedule.
- We didn't want to fall behind so we worked late into the night.
- The problems we have had mean that we have fallen behind and won't meet the deadline.
'to fall down' means to go from a vertical position to the ground.
- The old oak tree in the garden fell down in the storm.
- I slipped on the sidewalk and fell down and hurt my back.
'to fall down' means an argument or an idea is not complete or doesn't work.
- That's where your argument falls down. What you say isn't logical.
- This is the difficult part where everyone falls down. Nobody knows how to do it.
'to fall for' means to fall in love with.
- He is besotted. He has really fallen for her in a big way.
- He fell for her the first time he saw her and asked her to marry him the second time!
'to fall for' also means to be taken in by a trick or something that is not true.
- That is such an obvious lie. Nobody is going to fall for that.
- He told me he was going to invest the money for me and I fell for it.