An original line from Stratfor's Decade Forecast: 2015-2025, published in February:
The world has been restructuring itself since 2008, when Russia invaded Georgia and the subprime financial crisis struck. Three patterns have emerged. First, the European Union entered a crisis that it could not solve and that has increased in intensity. We predict that the European Union will never return to its previous unity, and if it survives it will operate in a more limited and fragmented way in the next decade.
Why is simple past used first, and then present perfect but not simple past again? It seems to break grammar conventions for consistency of tense.
Best Answer
The simple past talks about a completed action at some point in the past, i.e., the time before right now. So mark a point on the timeline before right now, and that's when the European Union entered the crisis.
The second past is not quite so simple. The first relative clause "(that) it could not solve" modifies "crisis," and the modal verb "could" brings the aspect of possibility (or rather with "not," of impossibility). The past sense of "could not solve" tells us that given what we know now, once the European Union entered the crisis, it would turn out to be unfixable. So the meaning is that the European Union at some point in the past entered an unsolvable crisis.
The second relative clause "that has increased in intensity" also modifies "crisis" but its verb in the present perfect which contemplates not just a point in past time, but any past time up to right now. Since we can't talk about the crisis before it happened, this present perfect is about the time interval between the point the European Union entered the crisis up to the present time. And during that interval things have only gotten more intense.