Idioms use language metaphorically rather than literally. When Helen said to Alice 'I've been in your shoes' she meant she had been in the same position as Alice (in having split up with her boyfriend). That's the metaphorical meaning, rather than the literal meaning 'I've been wearing your shoes Alice'.
Idioms are also fixed groups of words so you can't change the wording of an idiom. For example, you can 'keep something under your hat' (which means 'keep something a secret') but you can't 'keep something below your hat'.
talk through (a hole in) your hat
talk about something without understanding what you are talking about
He was talking through a hole in his hat. He used a lot of computer jargon but you could tell he didn't really know the first thing about IT.
no longer in fashion, not modern or exciting
Kids today think books are boring old hat. They think they can do read anything they need on their computers but I don't think you can ever replace the feeling of a real book in your hands.
throw your hat into the ring
announce that you are going to enter a competition or election
After she'd been a senator for so many years, no-one was surprised when Hilary Clinton threw her hat into the ring.
pull something out of the hat
do something unexpected which suddenly improves a bad situation
The interview was going really badly but when they asked me the final question about my ideas for the company, I managed to pull something out of the hat, and they offered me the job!
take my hat off to him/her
My grandma's 75 years old and she's just climbed Mount Everest. I take my hat off to her! I just hope I'm as active as she is when I'm old.
at the drop of a hat
do something immediately without stopping
I can't just go to the cinema with no warning. I need to get a baby-sitter first. I'm not like you young single ones who can do anything they like at the drop of a hat.
be all mouth (and no trousers)
talk a lot about doing something but never actually do it
He's been saying for years that he's going to buy a flash new car but we still haven't seen one. He's all mouth and no trousers.
get your knickers in a twist
become confused, worried or annoyed about something
It's no big deal. Don't get your knickers in a twist. It's only dinner with a few friends.
speak off the cuff
say something without having prepared or thought about your words first
She was fantastic. She just stood up and spoke off the cuff - no notes or anything.
roll up your sleeves
get ready for some hard work
OK, it's time to roll up our sleeves. We've only got 2 more days till this report has to be ready. Let's get to work!
walked out on him
left him (after being married to him)
he's got no backbone
he doesn't have a strong character
severe and sudden
some fresh air
clean and cool atmosphere found outside rather than inside a room or house
demand forcefully, especially when another person opposes the idea
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