This is a practical oversimplification which will be right most of the time. For more detailed information see Mastery of the Tenses!
"would do"
For repeated actions and habits in the past. Often in stories.
- When I got home my mum would always make me tea.
- He would always do his homework at the last possible moment.
- They would come to our house at Christmas.
- She would smoke 60 cigarettes a day!
Used in the Consequence clause when it is improbable or impossible. NOT IF CLAUSE (generally).
In Second Conditional sentences
- If I were you, I would go.
- If she was nice, she would invite us.
- They would be so excited if they won!
In Mixed Conditional sentences
- I would be so happy if she hadn't done that yesterday.
- We would all speak English, if we had studied harder.
- They wouldn't be here, if we hadn't invited them.