Learn Hardcore Korean: oneul meogeul eumsigeuro saelleodeureul junbihaesseo.

오늘 means “today.”
먹을 음식 is a noun phrase “food to eat,” made by attaching the future modifier -을 to 먹다
- 음식.
The particle -으로 here marks “as” or “for the purpose of.”
Put together, 오늘 먹을 음식으로 literally means “as the food we’ll eat today” or more naturally “for today’s meal.”

Why is it 먹을 음식 and not 먹는 음식 or 먹을 거?

먹을 음식 uses the future attributive form -을 to express something you plan to eat.
먹는 음식 (present modifier) would imply “food that is being eaten” or a habitual action.
먹을 거 (“thing to eat”) is more colloquial and less specific than 음식. You could say 오늘 먹을 거로 샐러드를 준비했어, but 음식 sounds more natural when talking about a meal.

How do you form the future modifier 먹을?

Attach -ㄹ to verbs ending in a vowel, or -을 to verbs ending in a consonant.

먹다 ends in the consonant ㄱ, so you add -을 → 먹을.
For a verb like 가다, which ends in a vowel, you add -ㄹ → 갈.

What role does 샐러드를 play and why -를?

샐러드 (salad) is the direct object of 준비했어 (prepared). The particle -를 marks that role. Since 샐러드 ends in a vowel sound, we use -를 rather than -을.

Why use 준비했어 instead of 만들었어?

준비했어 (“prepared”) covers the whole process: planning, gathering ingredients, assembling, etc.
만들었어 (“made”) focuses strictly on the act of making.
Use 준비했어 when you want to emphasize that you got everything ready for the meal, not just that you cooked or assembled the salad.

What level of politeness is 준비했어?

준비했어 is informal/casual past tense, typically used with close friends, family, or people younger than you.

To make it polite, you’d say 준비했어요.
For a very formal tone, use 준비했습니다.

Could I reorder the sentence differently?

Korean generally follows Subject-Object-Verb order, and modifiers come right before the noun they describe.

You could say 샐러드를 오늘 먹을 음식으로 준비했어, but it’s more natural to keep the time and modifier together at the beginning as 오늘 먹을 음식으로 샐러드를 준비했어.