Learn Polish: W niedziele ojciec grywał z nami w tenisa. - On Sundays father used to play tennis with us.

Both mean he played in the past, but grywał is a frequentative verb (the habitual form of grać). It strongly emphasizes that this was a recurring, habitual action over a long period, translating perfectly to "used to play" or "would play." While grał could mean he played once or was in the middle of playing, grywał makes the repeated nature of the habit unmistakable.

Why is it w niedziele instead of the more common w niedzielę?

W niedzielę is singular accusative and means "on Sunday." Here, we use the plural accusative w niedziele to mean "on Sundays." This plural form perfectly matches the frequentative verb grywał because the action was a habit that happened repeatedly across multiple weekends.

Since tenis is a masculine inanimate noun, shouldn't its accusative form just be tenis instead of tenisa?

Normally, yes! Masculine inanimate nouns usually look exactly like their nominative form when in the accusative case. However, there is a special exception for sports and games. When using the phrase grać w (to play), the sport or game takes the masculine animate ending -a. That is why it is grać w tenisa, w golfa, or w pokera.

Is there a specific reason to use ojciec here rather than tata?