Learn Italian: Il fatto che il complottista diffonda la diceria è grave. - The fact that the conspiracy theorist spreads the rumor is serious.

This highlights the core rule for il fatto che! We use the subjunctive here because the main clause (è grave) expresses a subjective judgment or emotional reaction. You aren't just objectively reporting that he spreads the rumor; you are evaluating that fact as 'serious.' If you were just stating an objective consequence, like 'The fact that he spreads the rumor makes him famous' (Il fatto che diffonde la diceria lo rende famoso), you would use the indicative.

The word complottista ends in '-a'. Why is it preceded by the masculine article il?

Nouns ending in -ista that refer to people (like complottista, turista, or giornalista) have the exact same form for masculine and feminine in the singular. You rely on the article to show the person's gender. Here, il complottista tells us this is a male conspiracy theorist. If it were a woman, it would be la complottista.

What is the base verb for diffonda?

The infinitive is diffondere (to spread or diffuse). While its present subjunctive (diffonda) follows the regular pattern for an -ere verb, it's worth noting for future reference that its past participle is irregular: diffuso.

I usually see voce used for 'rumor'. What is the difference between voce and diceria?

Voce (literally 'voice') is the most common and neutral way to say 'rumor' in Italian, often used in the phrase gira voce che (word is going around that). Diceria, used in this sentence, carries a stronger negative connotation; it specifically refers to malicious gossip, hearsay, or an unfounded tale.