Konuşulacak konuları ajandada işaretledim.

It’s a future participle that turns the verb into an adjective meaning “to be X-ed.”

• Breakdown: konuş- (to talk) + -ul- (passive) + -acak (future participle) → “that will be talked about / discussed.”
• So konuşulacak konular = “topics that will be discussed,” not a finite verb like “will be discussed.”

The passive makes it impersonal/generic: “to be discussed (by some people).”

• To include the agent, use a genitive + participle:
  • “the topics we will discuss”: konuşacağımız konular
  • “the topics I will discuss”: konuşacağım konular
• Your sentence could be made active: Konuşacağımız konuları ajandada işaretledim. (same meaning, now explicitly “we”.)

Accusative marks a specific/definite direct object.

• konuları = “the topics,” a known/identified set. That’s why it takes -ı.
• Without -ı (just konular) the meaning is non-specific (“topics” in general): Konular işaretledim would sound odd here because you’re clearly referring to a specific list.

Because vowel harmony looks at the last vowel before the suffix, and plural -lar changes that.

• konu
  • -lar → last vowel is now a.
• Accusative 4-way harmony after “a” is -ı: konu-lar-ı → konuları.

It’s the locative case: -da/-de = “in/on/at.”

• ajanda (planner/agenda book) + -da → “in the planner.”
• You’re saying where you marked them: in the planner.

Both are possible, depending on context.

• ajandada = “in the planner” (could be shared, the company planner, etc.).
• ajandamda = “in my planner” (possessive -m + locative -da): ajanda-m-da.
• If you mean your own notebook, ajandamda is more natural.

Not exactly.

• ajanda = a physical planner/diary (where you write plans).
• gündem = the agenda of a meeting (the list of items to discuss).
• “I marked them on the agenda (of the meeting)” would be: Gündemde işaretledim.

Generally no. işaretlemek conceptually marks something on/in a surface, so the locative -da is natural: ajandada.

• Use dative -ya/-ye with verbs of writing/adding into: ajandaya not aldım / yazdım (“I wrote into the planner”).
• For marking, stick with ajandada (on/in the planner).

Yes, it’s grammatical: Konuşulacakları ajandada işaretledim = “I marked those (things) that will be discussed.”

• It’s more compact but a bit more formal/elliptical; the original with konuları is clearer in everyday speech.

Turkish is flexible. The default is object–place–verb, but you can move elements for emphasis.

• Konuşulacak konuları ajandada işaretledim. (neutral)
• Ajandada konuşulacak konuları işaretledim. (emphasis on location)
• Konuşulacak konuları işaretledim ajandada. (end-focus on location; less common in writing) The finite verb typically stays at the end in neutral order.

Turkish uses participles, not relative pronouns, to form noun modifiers.

• English “topics that will be discussed” → Turkish participle phrase konuşulacak
  • noun konular.

Two harmonies apply:

• Vowel harmony picks -da vs -de: last vowel is back (a, ı, o, u) → -da; front (e, i, ö, ü) → -de. ajanda ends with “a” → -da.
• Consonant voicing gives -ta/-te after a voiceless consonant. Since ajanda ends in a vowel, we keep -da.

Yes; görüşmek/görüşülmek is often more formal/meeting-like (“discuss”).

• Görüşülecek konuları ajandada işaretledim. sounds very natural for meeting agendas.
• Konuşmak is more general “talk.”