有關英文名詞單複數的問題?

Some nouns have no plural forms.In some abstract nouns, we often use " a piece" and "some pieces" to count them.;eg:-evidence; We've "prove by evidence", "be evidence"; There were evidence(s) of struggling action on the body.
"were found two pieces of evidence(s)""were found some pieces of evidence(s)"
Some nouns are always plural;eg:-glasses, eye-glasses=spectacles;He can't read without spectacles. Or a pair of spectacles ; two pairs of spectacles.
Some nouns are always plural:-eg:-scales=a pair of scale(s), two pairs of scales; being simple balance or instrument for weighing.
We often use "a piece" and "some pieces " to count them;
eg:-2 pieces of scales; some pieces of scales".

1. 有兩項證據＝There are two pieces of evidence. (evidence為不可數名詞)
There are evidences that somebody has dug up the ground.（此時evidence為可數名詞，意思是［跡像］。

２.兩副眼鏡＝ two pairs of glasses
一副眼鏡= a/one pair of glasses
三塊玻璃= three panes of glass

3. 一台天平= a pair of scales
兩台天平= two pairs of scales