You already know how to combine modal verbs with the passive voice in the present tense. To talk about historical rules, past events, and former working conditions, simply shift the modal verb into the simple past tense.
Past passive in main clauses
In a main clause, the modal verb takes the second position and is conjugated in the simple past (for example, moest for "had to" or kon for "could"). The verb worden and the past participle are pushed to the very end of the sentence, commonly in the order worden + [past participle].
| According to the boss, the wages had to be paid to the workers. | Volgens de baas moest het loon aan de arbeiders worden uitbetaald. |
| The large machine could not be operated by one worker. | De grote machine kon niet door één arbeider worden bediend. |
If the subject is plural, use the plural past tense form of the modal verb, such as moesten or konden.
| The working conditions had to be improved quickly. | De werkomstandigheden moesten snel worden verbeterd. |
| In the industry, sick workers could just be fired. | In de industrie konden zieke arbeiders zomaar worden ontslagen. |
Subordinate clauses
Just like in the present tense, when a past passive construction appears in a subordinate clause (such as after als or omdat), all the verbs are pushed to the end of the clause.
A common and natural word order for this verb cluster at the end of the clause is [past modal] + worden + [past participle].
| If the machine could not be operated, the worker was fired. | Als de machine niet kon worden bediend, werd de arbeider ontslagen. |
| They had to strike because the factory would be closed. | Zij moesten staken omdat de fabriek zou worden gesloten. |
