Both USED TO and WOULD describe past habits or repeated actions. USED TO can also describe past states. WOULD only describes repeated actions (not states).
Quick tip: USED TO = past habits OR states. WOULD = past habits ONLY (repeated actions). For states (live, be, have), use USED TO. For repeated actions, both work!
USED TO = Past habits or states
Use USED TO for past habits (repeated actions) OR past states (situations that were true). Can use for both actions and states.
I used to smoke. (past habit - repeated action)
I used to live in Paris. (past state - situation)
She used to be shy. (past state - characteristic)
We used to go there every summer. (past habit)
WOULD = Past habits only (repeated actions)
Use WOULD only for past REPEATED ACTIONS (habits). Cannot use for past states. Only for actions that happened many times.
I would visit my grandmother every Sunday. (repeated action)
She would always arrive late. (repeated action)
We would go to the beach every summer. (repeated action)
Cannot say: I would live in Paris. (state - use used to)
When to use which
USED TO = past habits OR states. WOULD = past habits ONLY (repeated actions). For states (live, be, have), use USED TO. For repeated actions, both work, but WOULD is more formal.
I used to live there. (state - only used to)
I used to/would go there often. (habit - both work)
She used to be happy. (state - only used to)
She would/used to visit every week. (habit - both work)
Common mistakes
Not "I would live in Paris." — say "I used to live in Paris.". WOULD cannot be used for past states. Use USED TO for states (live, be, have)..
Not "I used to would go there." — say "I used to go there. OR I would go there.". Don't use USED TO and WOULD together. Choose one: used to OR would..
Practise it
Past state (live, be, have)? USED TO only. Past habit (repeated action)? Both USED TO and WOULD work. WOULD is more formal.