Easierは形容詞です。形容詞easyの比較級ですね。

 

That test was easy.

That test was easier.

That test was the easiest.

 

More easierは間違っています。最上級のeasiestではなかったら、まだeasierです。

 

 

Easierよりeasierの場合は”even”等を使えます。

例えば、

Level 4 was easy.

Level 3 was easier.

Level 2 was even easier (than level 3).

Level 1 was the easiest.

 

Easilyは副詞です。

 

You can easily do this.

You can more easily do this.

You can most easily do this.

 

You can move this easily with a cart.

You can move this more easily with a car.

You can move this most easily with a truck.

 

 

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。