Learn German: Der Sprengstoff hätte das Double beinahe erwischt.

In German, events that almost happened in the past but didn't (near misses) are expressed using the past subjunctive (Konjunktiv II) along with fast or beinahe. While English uses the simple past for this, German is more literal about the unfulfilled condition, essentially saying the explosives "would have almost caught" the double.

The English translation uses the plural "explosives", so why is Sprengstoff singular in German?

Der Sprengstoff refers to the explosive material or substance itself. Because it acts as an uncountable mass noun in this context, German uses the singular form. You will rarely see it in the plural unless referring to distinct types of explosives (die Sprengstoffe).

Could I put beinahe earlier in the sentence, for example right after hätte?

Yes, but it changes the emphasis. Adverbs like beinahe typically go right before the part of the sentence they modify. Placed right before erwischt, it emphasizes that the action of catching almost happened. If you put it before das Double (... hätte beinahe das Double erwischt), it sounds like the explosives almost caught the double, but perhaps actually caught someone else instead.

The verb here is erwischen. How is that different from a standard word for catching, like fangen?