Learn Portuguese (Portugal): Se o árbitro visse a falta, o jogo seria diferente. - If the referee saw the foul, the game would be different.

Because this is a hypothetical situation. When we use se (if) to talk about imaginary or unlikely scenarios, we must follow it with the Imperfect Subjunctive. visse is that form for the verb ver (to see).

Could I say o jogo era diferente instead of o jogo seria diferente?

Yes, absolutely! In spoken Portuguese, people very often replace the Conditional (seria) with the Imperfect Indicative (era) in everyday conversation. Both are completely correct here and mean the exact same thing.

I know falta usually means a lack or shortage of something. Why does it translate to 'foul' here?

In sports contexts, uma falta specifically refers to a foul or rule violation. It comes from the same general idea of a fault or failing to follow the rules, even though in everyday life you will usually see it meaning a lack or absence.

Is there a trick to remembering how to form the Imperfect Subjunctive like visse?

Yes! A great trick is to start with the they/plural form of the normal past tense. For ver, they saw is viram. If you drop the -ram ending and add -sse, you get visse. This pattern works for almost all Portuguese verbs!