Past year x Last year - Qual a diferença?

Briefly, what's the difference between "past" and "last"

What's a Past week and a last week?
What's past months and last months
What's last years and Past years
What's the difference in saying: In the last lessons x In past lessons?

Are they the same? Are they interchangeable?

Thank you so mcuh for you help!

Please check the difference out here.