Learn Turkish: Biliyorum ki izleyiciler çok mutlu. - I know that the viewers are very happy.

The root is the verb izlemek (to watch/follow). When we drop the infinitive, we get izle-. Since it ends in a vowel, we add the buffer letter -y-, followed by the agentive suffix -ici (one who does the verb). This gives us izleyici (viewer). Finally, -ler makes it plural.

Why does the agentive suffix take the form -ici here instead of -ucu?

The suffix -(y)ıcı / -(y)ucu follows four-way vowel harmony. The last vowel in izle- is an e, which dictates an i in four-way harmony, making the suffix -ici. If the verb were koşmak (to run), the o would require an u, giving us koşucu (runner).

Using 'ki' to mean 'that' feels like English syntax. Is it natural to say it this way instead of using an object participle like 'olduğunu'?

Yes, it is very natural, especially in spoken Turkish! While the traditional Turkish structure uses participles (like izleyicilerin mutlu olduğunu biliyorum), using the Persian borrowing ki lets you link two clauses exactly like the English 'that'. It is grammatically correct and often adds a conversational or slightly dramatic tone.

Why is 'I know' translated with the continuous tense (biliyorum) rather than the aorist/simple present (bilirim)?