Learn French: On se doute qu'il a reproduit le tableau le plus célèbre. - We suspect that he reproduced the most famous painting.

The verb se douter que means "to suspect" or "to have a feeling that" something is true. Because it expresses a strong probability or belief, it takes the indicative. If the sentence used douter que (without the reflexive se), it would mean "to doubt that" and would require the subjunctive, because it expresses uncertainty.

Does se douter have anything to do with doubting?

Not really! While douter means "to doubt," making it reflexive (se douter) flips the meaning to almost the opposite: "to suspect," "to imagine," or "to guess" that something is the case.

Why are there two le words in le tableau le plus célèbre?

In French, to form a superlative like "the most famous," you use le plus, la plus, or les plus followed by the adjective. When the adjective comes after the noun (as most do, including célèbre), you must use the definite article twice: once for the noun (le tableau) and once for the superlative (le plus célèbre).

The English translation says "We suspect," but the French uses on. Why not nous?