予定だ means 'plan to' or 'intend to'. It expresses a concrete schedule or intention that the speaker has arranged or decided. You use it when you have a specific plan, like a meeting, a trip, or a task. It is more definite than just 'want to' (たい) because it implies a real arrangement or decision.

When the action is a verb, put it in its dictionary form before 予定だ: 行く予定だ means 'I plan to go'. For nouns (like 'meeting' or 'trip'), you say 会議の予定だ, meaning 'I have a meeting scheduled'. For な-adjectives, use な before 予定 (e.g., 静かな予定? No, that's not common; actually, for な-adjectives you say 予定だ after the noun or use な+予定? Wait, better: for な-adjective + noun, you say な予定, but for a plan to be something, you say になる予定だ. E.g., 医者になる予定だ. If you mean 'a quiet plan', that's odd. So the rule is: attach to verb dictionary form, or for noun events use の予定だ. For adjectives describing the plan itself, use の予定だ after the noun, not adjectives.

Remember: 予定だ is often used in polite speech as 予定です. In casual conversation, you can say 予定. It is neutral in register, used in both speaking and writing.