Question

the shirt washes easily.为什么不是被动形式be +过去分词？

Answer 1

英文里有些动词是用主动的形式表示被动的意义的，这类动词主要有以下一些：
一、联系动词。如：
The flower smelt sweet.
The soup tasted a little too salty.
Your reason sounds reasonable.
The material has worn thin.
The door blew open.
The news has proven true.
二、某些与can't、won't连用的动词，如：
It can't move.
The windows won't open.
The rusty screw wouldn't come lose.
The pen won't write.
Meat won't keep long in such hot weather.
三、表示主语的特征或性质的动词，如：
This knife cuts well.
These books sell well.
The pen writes smoothly.
The cloth washes well.
This kind of paper tears easily.