There are several flavors of bad things that can happen to our class. It is good to have a strategy in mind to deal with them.
The general strategy for dealing with all of these problems is the same. Plan ahead by acquiring:
Of these, the Reader's Edition GNT is obviously the more important. If you are reading Greek by candlelight, you simply MUST have a copy of the GNT. A textbook is not as critical.
Plummer and I share a different approach to teaching, of course. One of my lessons will be named something like Acts 2:30-35. Plummer's lesson will be called something like SECOND AORIST INDICATIVE VERBS.
But still, in terms of our understanding of what makes Greek tick—the grammar of NT Greek—Plummer and I are extremely compatible.
The good news about Machen is that his book is 99 years old, and is in the public domain. You can get it for free. Thousands of students have learned Greek using Machen's book over the course of the past century.
The bad news about Machen's book is that it is 99 years old. We simply have a better understanding of Koine Greek now than we did a century ago. This is particularly true when it comes to verbs.
It can't hurt to download Machen right now. There are no downsides. If you lose the Internet, but your computer still works, then you can use Machen. Unzip the file and put all the contents in some folder on your computer. There is one PNG file per page of Machen's book. It is inconvenient to use...but as I say, it is free.
If you lose power as well as the Internet, then your computer is also inoperative. Having downloaded Machen won't do you any good. You will need to already have a hard copy textbook...and for that, you want Mounce.
DO NOT TRY TO PRINT OUT ALL 914 FILES that go to make up Machen's book. By the time you do that, it will probably turn out to be less trouble—and maybe even cheaper, since you will not want 914 loose sheets of paper; you will need to purchase one or more binders as well!—to simply purchase a copy of Mounce's book.
If you DO purchase Plummer, and I am still available to you by Zoom/Internet, then you can of course use it at any time. But you can also just leave it on your shelf as a reference book, and save it there primarily as a hedge against bad things happening.