Life in an isolated lighthouse involves constantly watching the sea and the weather.
| I was looking at the horizon when the dense fog arrived. | Eu estava a olhar para o horizonte quando o nevoeiro denso chegou. |
When observing these changing conditions from afar, you naturally make assumptions about what is happening out on the water.
Expressing Probability with "Dever de"
In Portuguese, the verb dever often expresses obligation (should/must do something). However, when you want to express a strong deduction or probability based on evidence—like assuming someone "must be" lonely or a ship "must be" English—you use dever, followed by the preposition de, and an infinitive verb.
| must (probability) | dever de |
| In fact, the lighthouse keeper must be very lonely. | De facto, o faroleiro deve de estar muito sozinho. |
| The vessel must be English. | A embarcação deve de ser inglesa. |
This structure is very common when speculating about the harsh coastal weather, such as an incoming gale (a ventania).
| With this gale, the waves must be huge. | Com esta ventania, as ondas devem de estar enormes. |
| The dark sea must be roaring. | O mar escuro deve de estar a rugir. |
Probability with Continuous Actions
If you are assuming that an action is currently ongoing, you simply combine dever de with the continuous aspect (estar a + infinitive).
| The seagulls must be looking for the lighthouse keeper. | As gaivotas devem de estar a procurar o faroleiro. |
Speculating About Intentions and Future Events
You can also use this periphrasis to speculate about what others want to do, especially when faced with extreme situations like an imminent shipwreck (o naufrágio).
| the shipwreck | o naufrágio |
| They must want to avoid the shipwreck. | Eles devem de querer evitar o naufrágio. |
| Since the storm is imminent, the vessel must return. | Visto que o temporal é iminente, a embarcação deve de regressar. |