Learn French: L'historien se doute qu'il a attribué ce tableau à un autre peintre. - The historian suspects that he attributed this painting to another painter.

The verb se douter que means 'to suspect' or 'to have a strong feeling that'. Because it expresses a probability or a strong belief rather than uncertainty, it is followed by the indicative. This contrasts directly with douter que (to doubt), which expresses doubt and requires the subjunctive.

Does se douter have anything to do with doubting?

Not quite! While douter means 'to doubt', making it reflexive (se douter) flips the meaning almost to its opposite: 'to suspect' or 'to guess'. So instead of doubting that something is true, the historian suspects it is true.

Why use qu'il a attribué instead of using an infinitive construction like d'avoir attribué, since the subject is the same in both clauses?

With many verbs, like vouloir or regretter, you usually must use an infinitive when the subject is the same in both parts of the sentence. However, se douter que is typically followed by a full clause with a conjugated verb, even when the subject is the same.

Why is the preposition à used after attribué?

In French, you attribute something to someone using the structure attribuer quelque chose à quelqu'un. The preposition à introduces the indirect object receiving the attribution, which in this case is the other painter (un autre peintre).