Simpel:
Tegenwoordige tijd -> am/are/is + werkwoord + ed OF 3e vorm (or ww)
*The windows are cleaned every week.
*Many cars are made in that factory. (niet: maked!)
Verleden tijd -> was/were + werkwoord + ed OF 3e vorm (or ww)
*The man was robbed by the thief last night.
*Many items were stolen in that city back in the days.