"Het hemd werd gewassen."
Translation:The shirt was being washed.
I don't really agree with that website, when it comes to this. The thing is that you will rarely find "The shirt was washed" on its own. Instead you might see:
- The shirt was washed yesterday - Het hemd is gisteren gewassen.
- I wanted to do laundry, but the shirt was already washed - Ik wilde de was doen, maar het hemd was al gewassen. (past perfect)
I can't think of a common situation where you would use "worden" in this case, but maybe there is one. As you can see, there is no one-to-one correspondence between tenses.