Learning a language takes time and effort, especially if it is not your first language. Even if it is your mother tongue, you still need time and effort to master it. Language is forever changing. Colloquial expressions are often acceptable in informal writing. The more you learn, the more you will know when to use them or not to use them in your writing or speaking.
Slow on the uptake: slow to understand.
e.g. I'm a bit slow on the uptake. Can you explain it once more?
Monkey business: foolish behavior.
e.g. Behave yourself! Stop this monkey business of yours!
Pile on the agony: exaggerate.
e.g. Don't pile on the agony; it's not as bad as it looks.
In the bag: pretty certain.
e.g. I tell you what: your promotion is in the bag.
Are you with me?: understand or agree with me.
e.g. I've been explaining this for an hour. Are you with me?
Have someone by the short hair: have control over; have someone at a disadvantage.
e.g. Not having adequate preparation will let your opponent have you by the short hair.
Hell for leather: at a reckless speed.
e.g. Some teenagers drive their cars hell for leather; they endanger not only their lives but also those of others.
In a jiffy: soon.
e.g. The manager will see you in a jiffy.
e.g. Will you stop that half-baked behavior!
Fall over oneself: too eager.
e.g. He fell over himself to get that job.
Finger in the pie: share of responsibility.
e.g. He wants to have his finger in every pie that we are going.