Learn Hardcore Japanese: syuumatu ni ani to kouen de yakyuu wo simasu. - I play baseball with my older brother at the park on the weekend.

に marks a specific point in time. When you want to say “on the weekend,” you attach に to 週末. Without に, it’s still understandable colloquially, but grammatically に tells us “that time” is when the action happens.

Could I use は instead of に after 週末?

Yes, you can say 週末は instead. That makes 週末 the topic (“As for weekends…”), which might imply contrast (e.g. weekends vs. weekdays) or set up background information. Using に simply pins down the time without topic nuance.

What does the particle と after 兄 indicate?

と here means “together with.” 兄と＝“with (my) older brother.” Whenever you want to express doing something jointly with someone, you use と after the person.

Why is 公園 followed by で instead of に? What’s the difference?

で marks the location where an action takes place (“at/in the park”). By contrast, に can indicate destination (“to the park”) or existence (“is at the park”). Since you’re doing baseball at that spot, you use で.

Why do we use を after 野球? It doesn’t feel like a “direct object” in English.