Learn Italian: Non sappiamo perché quell'attore affascinante è ancora single. - We do not know why that charming actor is still single.

We use se (if/whether) to report Yes/No questions, like 'We don't know IF he is single.' But since the original question here asks 'Why?' (perché), it is a Wh- question. For these, we just keep the question word (perché, dove, quando, etc.) to connect the sentences instead of using se.

Since the sentence starts with non sappiamo (we don't know), shouldn't the verb be in the subjunctive mood (sia) instead of the indicative (è)?

In strict formal grammar, indirect questions introduced by verbs of doubt or not knowing can take the subjunctive (sia). However, in everyday spoken Italian, it is extremely common and perfectly acceptable to use the indicative (è) for indirect questions.

Why is the word for 'that' written as quell' instead of quello?

The demonstrative adjective quello changes its ending based on the noun it precedes, using the exact same rules as the definite article ('the'). Because attore starts with a vowel, it would take the article l' (l'attore). Therefore, quello drops its final vowel and takes an apostrophe to become quell'.

Why doesn't the adjective affascinante end in an to match the masculine noun ?