For an example of the deliberate miss-use of a past tense for poetic reasons, there's the first verse of "Cân y Melinydd" where the use of the wrong past tense retains the meter & scansion:

Mae gen i dy^ cysurus
A melin newydd sbon
A thair o wartheg brithion
Yn pori ar y fron

I have a cosy cottage ,
A newly builded mill.
And three contented dappled cows
Are grazing on the hill.

Cheers
Nigel