Let alone
Is used to show that if one thing is difficult or impossible, then something else is even more difficult or impossible. It emphasizes that the second thing is much harder than the first.

Examples:
1- When his aunt asked if he liked the sweater she knitted, he said it was beautiful, even though it wasn’t his style, to make her happy.
2- He used a white lie to explain why he was late to meet his friend, saying he got stuck in traffic when he had just been running late.

The Elephant in the Room
The "elephant in the room" is a big problem or issue that everyone knows about but doesn’t want to talk about.

Examples:
1- At lunch, the elephant in the room was that one friend had just lost their job, and everyone knew it, but they didn’t want to make them feel uncomfortable by bringing it up.
2- At community meetings, the elephant in the room might be systemic racism, which can be an uncomfortable topic for some but is crucial to address for real change to occur.

On edge
Feeling very nervous, tense, or anxious, often because something stressful is happening or you’re waiting for something important to happen.

Examples:
1- The quick rise of the new social media app has made the older platforms nervous, afraid they might lose users.
2- The controversial law passed last week has put many civil rights activists on edge, fearing it could limit freedoms.

Cold Fish
A "cold fish" is a person who lacks emotional warmth and often seems unfriendly, detached, or indifferent towards others.

Examples:
1- The politician's debate performance was criticized because he appeared as a cold fish, lacking passion or connection with the audience.
2- When I told my uncle about my promotion, he just nodded and continued reading his newspaper. He's always been a bit of a cold fish.

Naive
Describes someone who is too trusting or innocent, often because they lack experience or knowledge about how things really work. A naive person believes things too easily or trusts people too readily.

Examples:
1- Maria was naive to believe that everyone she met at the party had good intentions. She didn’t realize that some people might take advantage of her kindness.
2- Some activists were naive in thinking that a single campaign would be enough to change environmental policies. They didn’t anticipate the long-term effort needed.

Over the moon
Extremely happy, delighted, or pleased about something.

Examples:
1- People in both East and West Germany were over the moon when the Berlin Wall fell, symbolizing the end of decades of division.
2- The parents were very happy when their child took their first steps and they recorded the special moment on camera.

Fishy
When something seems suspicious, strange, or not quite right, it can be described as "fishy.

Examples:
1- There was something fishy about the sudden disappearance of key documents related to the trial of a famous historical figure.
2- There’s something fishy about the fact that everyone in the office is getting sick at the same time, right after that new cleaning product was introduced.

Happy-go-lucky:
describes someone who is cheerful and carefree, not worrying much about problems or what might happen in the future.

Example:
1- Despite the difficulties of leading a country during the war, Winston Churchill often showed a carefree attitude in private, making jokes to cheer up the people around him.

2- When her homemade cake fell apart before the party, Sarah’s cheerful attitude turned it into a “cake decorating contest” where guests could help fix it.