Logical Deductions with trebuie să
You already know how to use trebuie să to express obligation (what someone "must" do). However, you can also use it for epistemic deduction—stating what "must be" true based on logical evidence.
Just like when expressing obligation, trebuie remains fixed, and the verb that follows să conjugates in the subjunctive mood.
Present Deductions
To state what must be true right now, use trebuie să followed by a present tense verb.
| The woman could not drag the body alone, so she must have an accomplice. | Femeia nu putea târî corpul singură, deci trebuie să aibă un complice. |
| As an inspector, I believe that the blackmail must be the accomplice's motive. | Ca inspector, cred că șantajul trebuie să fie motivul complicelui. |
Past Deductions
When piecing together evidence, you often need to deduce what must have happened in the past.
To express "must have," use the past subjunctive. As you learned previously, this is formed with să + fi (which never changes) + the past participle of the action verb.
| The suspect must have dragged the corpse into the forest. | Suspectul trebuie să fi târât cadavrul în pădure. |
| The wife must have poisoned the coffee. | Soția trebuie să fi otrăvit cafeaua. |
"Must Have Been"
If you want to say that something "must have been" a certain thing, use the past subjunctive of a fi (to be): să fi fost.
| It must have been the poison. | Trebuie să fi fost otrava. |
When you use să fi fost to form the passive voice (meaning an action was done to the subject, like "must have been stabbed"), you add the past participle of the main verb at the end.
Remember that in the passive voice, this final past participle acts like an adjective and must change its ending to match the subject's gender and number.
| The autopsy confirms that the victim must have been stabbed. | Autopsia confirmă că victima trebuie să fi fost înjunghiată. |
Notice how înjunghiată ends in -ă to match the feminine singular victima.
| The glasses are on the table; the men must have been poisoned. | Paharele sunt pe masă; bărbații trebuie să fi fost otrăviți. |
Here, otrăviți ends in -i to match the masculine plural bărbații.
