Anyone reading this who knows better, please correct. The following is just my understanding as a beginner learner (albeit seasoned linguist ).
@JohnLever John, you mentioned it yourself, and that’s exactly the point: in the examples, it isn’t about past actions. It’s the English which is doing a great job at confusing us here.
So, Sir Cuthbert Higglesbottom comes out with: “The current government is complete rubbish.” (A statement about a present state of affairs.)
Now you want to report this via indirect speech. Sir Cuthbert Higglesbottom said that the current government …
The Welsh language doesn’t see a need to change the tense in what comes next. And why should it? You’re just going from direct to indirect speech, that’s all there is to it.
Whereas English has something called “sequence of tenses” or sometimes “backshifting”.
Sir Cuthbert says that the current government is complete rubbish.
Sir Cuthbert said that the current government was complete rubbish.
That the tense is changed in the second example has nothing to do with the statement itself (which remains, ahem, true in the present), but with the tense of the introductory clause. As also in the following:
Sir Cuthbert says that the previous government wasn’t much better.
Sir Cuthbert said that the previous government hadn’t been much better.
Sir Cuthbert says he will move to Andorra if things don’t get better.
Sir Cuthbert said he would move to Andorra if things didn’t get better.
Confusing, isn’t it? Trust me, it’s worse in German, where you’d start using the subjunctive in some of these … So Welsh has a beautiful simplicity here.
Edited to add: @johnwilliams_6 has overtaken me and explained it a lot more concisely!