Can I use "'d" instead of "would" here?

Yes. The exercise accepts both forms. In "we'd leave", "'d" means "would" because it is followed by the base verb. Keep that verb unchanged when you shorten "would".

Could "used to" describe these repeated actions too?

Yes, "used to" can also describe past habits. Here, the instructions ask you to practise "would", so use that form. For a continuing past state, such as owning something, "used to" is the usual choice.

Home Grammar Used To ExercisesWould for Past Actions Exercises

Would for Past Actions Exercises

B1 Level

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These sentences look back at things people did again and again. Add would + the base verb: When we shared a flat, we'd leave notes on the fridge. The past setting tells you that would describes a memory here.

You don't need a past ending or an -s on the next verb. Would stays the same with every subject, too. That's one less change to remember 🙌 This page gives you repeated actions to practise; choosing between past actions and continuing states belongs to the comparison exercise.