It was closed

這句句子是被動句，指那個東西被關起來了。

Who closed the window? (verb in past tense)
Tom did.
The window was closed by Tom. (past participle in passive voice)


When was the shop closed yesterday? (adjective)
It was closed at 7:00 pm.

closed 可以做形容詞和過去分詞, 要看怎樣使用

Who close the gate?
The gate was closed by my father this morning, (closed 是過去分詞)

closed 在字典分開地列為形容詞, 形容狀態 (state) = 關閉的
This shop is closed on Sundays.
It is a closed meeting. (closed = restricted to a particular group of people)

It is closed.

這個 "closed" 可以說是 participle adjective

close 可以是形容詞
= 靠近的,親近的, 親密的
The church is close to our house.
Another example:~ a close friend

小心:
The shop is closed. (closed 是形容詞)
The shop is open. (open 是形容詞)

2015-07-15 01:05:09 補充：
It is closed by someone.
close 是瞬間動詞, closed 是過去分詞 past participle
句子是 present simple tense, in passive voice. 有 agent (someone)
is 是助動詞 auxiliary verb

It is closed
closed 不是動作, 是形容詞
形容 It (subject) 的狀態 (state), 是關閉的
is 是連綴動詞 linking verb.
總言之, 現在不是開著的,不是營業著的, 不知道何時關閉, 關閉多久

2015-07-15 11:04:36 補充：
我太大意, 謝謝 Julie 改正,

Who closed the gate?

因為關門動作發生在過去, 應用 past tense
問這句話時, 門是關閉的

2015-07-15 11:14:27 補充：
發問者寫:

It was closed.

closed 是 "形容詞"
句子是用 past simple tense

2015-07-15 21:28:40 補充：
Conversational English

Who closed the gate?
My father.

如果是我做
Me. / It was me. / I did it.

我寫這句 "The gate was closed by my father this morning," ~ stuffy!
改用 passive voice, 言及時間, 純是想說明 closed 是過去分詞
請見諒

I am interested.

這個 interested 也是用作 adjective (形容詞) 的 past participle (過去分詞).