i agree with Duncan. Rather than looking for a rule, learn the vocabulary. Whereas Jacob's rule makes a lot of sense, Tennis is always played on a court, even when it is on grass and outside.

Thanks a lot

Field: outside with grass and dirt. Court: inside and/or paved ground.

I'm sure there's an actual rule for this, but in my experience most racket games, games over a net, or inside sports, are played on courts. In the US they seem to use field for outdoor sports a lot. In the UK we normally call it a "pitch" (football pitch, cricket pitch). I think we call hockey "field hockey", so maybe that's a field (I'm sure I've always called it a hockey pitch)? My point is: it's probably easier to learn what the sport is played on for each case. But if it's inside it's often on a court and outside often on a field, or pitch.