Translation:When he would decide to do something, he would do it.
Assuming the context is repetition, 'used to' is roughly equivalent to 'would'. Maybe it emphasises habit a bit more.
However, both are clumsy in this sentence, in the first verb alone and especially when used twice. At the very least I'd use one then the other, but that still sounds wrong. I'd translate this as "Whenever he decided ..., he would ...". Repetition is implicit in "whenever" before an action.
Quando, decideva . . . . . , di fare qualcosa , lui la faceva.
When, he(she/it) decided, to do something, he did it.
Couldn't it be "When she decided to do something, he would do it", in the sense that was copying here actions?
Could be, and it could describe a few marriages I know :-) What's more, it makes lui necessary rather than optional - although I'd expect lei decideva ... too.
that's what I thought. Can't "fare" mean to make something as well as do something?
When did the conditional come into the answer. Could and Would are conditional as far as I know. Please explain.
... and one of the translations of the imperfetto denoting habitual or accustomed actions in the past - which is how Duo uses it here.