When playing detective and trying to piece together past events, you often need to express how certain you are about what happened. Swedish does this just like English: by combining a modal verb (måste, kan, or bör), the auxiliary verb ha (have), and the supine form of the main verb.
Possibility: Kan ha
If you are investigating a mystery and want to express that something is a possibility, use kan ha (may have / could have). It suggests that an event might have occurred, but you still lack definitive proof.
| We must investigate who may have opened the safe. | Vi måste utreda vem som kan ha öppnat kassaskåpet. |
| They may have left the fingerprint on the safe. | De kan ha lämnat fingeravtrycket på kassaskåpet. |
High Certainty: Måste ha
When the evidence points strongly to one unavoidable conclusion, use måste ha (must have). This expresses a very high degree of certainty about a past event.
| The perpetrator must have stolen the will. | Förövaren måste ha stulit testamentet. |
| The suspicion is that someone must have put the poison in the wine. | Misstanken är att någon måste ha lagt giftet i vinet. |
Logical Probability: Bör ha
If something is highly likely based on logic, expectations, or common sense, use bör ha (ought to have / should have). It indicates what you reasonably assume happened because it makes the most sense.
| There is an heir who ought to have read the will. | Det finns en arvinge som bör ha läst testamentet. |
| The perpetrator ought to have known where the hiding place was. | Förövaren bör ha vetat var gömstället fanns. |
Negative Deductions: Kan inte ha
To deduce that something was impossible, use kan inte ha (cannot have). Notice the word order: the negation inte comes immediately after the modal verb kan, right before ha.
| She cannot have hidden the poison in the hiding place. | Hon kan inte ha gömt giftet i gömstället. |