過去式was/were

過去式幾時要加was/were? E.g., This shopping mall is new.It was built Las year.E.g.,Our products were exported to different countries last year. 點解以上全部例子都要加was/were?雖然有人問過類似的問題,但我都係吾明!!!請解釋清楚!! Please,please!!!Thank you！
註解：我們在選擇該用那個時式的時候, 先留意句末的時間用字。
★ 如：yesterday, last night, last year, an hour ago,

three weeks ago, the other day, in1988( 過去了的年分).....等等那些表示過去已發生了的, 所以要過去式。
★★ 別外was/ were + past participle ( 過去分詞) = 被動的意思
★例： This shopping mall is new.It was built last year.這個購物商場是新的，它是於去年建築的。( 建築物是被人建造的)
★ I, She ,He or , 單數主持用＂was"
★ We, They, The children 等眾數主持用＂were"


圖片參考：https://s.yimg.com/rk/AD05634470/o/1118526210.jpg

Eg, This shopping mall is new.It was built las year.
was是動詞 built 也是
原則上一句用一個過去式即可
一個句子裡動詞只能出現一次或be+動詞ing.加to
只能用以下用法:
1.This shopping mall is new.It built last year.
(捨去was
2.This shopping mall is new.It was building las year.
(be+動詞ing
3.This shopping mall is new.It was to build las year.
(+to
1.Our products exported to different countries last year.
2.Our products were exporting to different countries last year.
3.Our products were to export to different countries last year.