Learn Turkish: Semineri verecek moderatör bugün meşgul. - The moderator who will give the seminar is busy today.

You're right that verecek is the 'he/she/it' form of the future tense. However, in Turkish, when placed directly before a noun (like moderatör), the -ecek / -acak suffix acts as a participle. It turns the verb into an adjective describing the noun. Because it comes before the noun instead of at the end of the sentence, it translates to 'who will give' rather than acting as the main verb.

Why is it semineri with the accusative suffix instead of just seminer?

Even though verecek is acting like an adjective here, it's still built from the verb vermek (to give) and still takes its own direct object. Because the moderator is giving a specific seminar, seminer needs the definite accusative suffix -i.

Can I use this same -ecek pattern to say 'the seminar that the moderator will give'?

No, this specific pattern only works when the noun you are describing is the one doing the action (the moderator is the one who will give). To say 'the seminar that the moderator will give', the object is being described, which requires a different structure: moderatörün vereceği seminer.

Where is the verb for 'is' in bugün meşgul?