Prepositions may be small, but they usually have a big impact. This is particularly true in the case of the effect of the preposition ‘de’ when paired -or not- with the verb ‘deber’.
Although in English both correspond to the verb must, in Spanish ‘deber’ expresses obligation while ‘deber de’ indicates deduction. Let’s take a look at their differences!
| Es una cena muy importante. Debes portarte bien. | /obligation/ |
You must behave. |
|
Mañana tienes un examen. Debes estudiar. |
/obligation/ |
You must study. |
| No tienes la campera puesta. Debes de tener frío. | /deduction/ |
You must be cold. |
| ¡Qué físico tienes! Debes de ser físicoculturista. | /deduction/ |
You must be a bodybuilder. |
Please note that in real life native speakers often don’t pay attention to these rules, so you can hear phrases like ‘No tienes la campera puesta. Debes tener frío’ (‘You’re not wearing a jacket; you must be cold’), which is not an obligation, but rather a guess even if it’s missing the preposition de.