How do I override the subscription block? I am developing a Drupal 8 website and I want to override the subscription block used by the Simplenews module. I know how to achieve that at Drupal 7, but the module is different now. For Drupal 7, I followed http://api.worldempire.ch/api/simplenews/theme%21simplenews-block.tpl.php/7-2.

I want to do the same for Drupal 8. How can I achieve it?
There is no exact equivalent currently. To override the actual block template for that block you can use 'block--simplenewssubscription.html.twig'. You can use the input and form templates to override other elements. I would suggest enabling debugging in Twig so you can see the template overrides.