Indoor は形容詞として使います。

 

Gymnastics is an indoor sport.

 

There is an indoor pool at the complex.

 

My cat is an indoor cat.

 

 

間違っている使い方は、

 

Let’s stay indoor today.

(should be “stay indoors”)

 

I practiced golf indoor yesterday.

(should be “practiced golf indoors)

 

 

Indoorsは副詞として使います。

 

Let’s stay indoors since the weather’s terrible today.

 

The team practiced indoors at the facility.

 

 

 
 

間違っている使い方は、

 

Table tennis is an indoors sport.

(should be “indoor sport”)

 

There is an indoors putting green at his house.

(should be “indoor putting green”)

 

 
英語の文法練習はこのテキスト・Understanding and Using English Grammar with Audio CDs and Answer Key (4th Edition)をお勧めです。文法の参考書は、こちらのガイド・Practical English Usageも実用的です。私も両方レッスンで使っています。