Things change refers to life. The things the human being does suffer changes and they even diappear.Mankind does not last for ever. The words on the pedestal refer to eternity , but this does not exist. Men can create art as a way to materialise their eternity , yet art can be affected by the passage of time. To illustrate this , the author refers to the figure of the desert. After reading the poem, we can see the remains of a piece of art buried in the sands of the desert. This means that the eternal work of mankind is affected by the natural passage of time. Nothing is left but the presence of the desert itself.
This is because at the beginning of the sentence there is already a past tense "did" so the verb after that would have to be in base form which would be "use to". It's a rule so I can't really explain much.
