Questions & Answers about To głupi zakład, oby nie grali.
Why is grali in the past tense when the English translation says "won't play", which is in the future?
This is the core rule for the particle oby (hopefully / if only). It must always be followed by the past tense form of the verb, regardless of whether your wish is about the present or the future.
Who exactly is the subject of grali in this sentence?
The ending -li tells us the hidden subject is a masculine personal "they" (oni), meaning the group making the bet includes at least one man. If the group consisted entirely of women, children, or non-humans, the verb would be grały.
Why use the imperfective grali instead of a perfective verb here?
Using the imperfective grali means the speaker hopes they won't engage in the general activity of playing at all. A perfective verb might focus on completing a single game, but here the speaker is simply wishing against the action taking place.
Why does the first clause use To głupi zakład in the nominative case instead of the instrumental case?
When you use the pronoun to by itself to mean "this is" (without the verb jest), the noun phrase that follows must stay in the nominative case. If you wanted to use the instrumental case, you would have to add the verb: To jest głupim zakładem.