Learn Hardcore Korean: toyoire sijangeseo gwaireul sayo. - On Saturday, I buy fruit at the market.

에 is a particle that marks a specific point in time, similar to on in English. So 토요일에 literally means on Saturday—it tells us when the action happens.

What’s the difference between 에 and 에서, and why is 시장에서 used here?

에 marks a destination or a point of existence (e.g., 학교에 가요 = “I go to school”).
에서 marks the location where an action takes place (e.g., 공원에서 놀아요 = “I play at the park”). Since buying fruit is an action happening at the market, we use 시장에서 (“at the market”).

Why does 과일 have 을 attached? What does 을 mean?

을 is the object particle used after a noun ending in a consonant. It marks 과일 as the direct object of the verb 사요. If the noun ended in a vowel, you’d use 를 instead.
Example:

책을 읽어요 (I read a book)
사과를 먹어요 (I eat an apple)

Why is the verb written as 사요 instead of 사다?

사다 is the dictionary (infinitive) form “to buy.”
To make it present tense polite, you drop -다 and add -아요 (because the stem vowel is ㅏ): 사-
- 아요 → 사요.
  This polite ending is standard in everyday conversation.

The subject is missing. How do we know who is doing the action?

In Korean, the subject is often omitted when it’s clear from context. If no subject is stated, it’s usually understood to be “I” (저 or 나) or “we” in a given conversation. You can add 제가 (“I”) or 나는 before the verb if you need to be explicit.

What level of formality does the ending -요 convey?

The -요 ending (해요체) is polite but not overly formal.

Use -요 with strangers, acquaintances, coworkers.
For very formal or official speech, you’d use -ㅂ니다/습니다 (e.g., 사습니다).
To close friends or younger people, you might drop -요 altogether (e.g., 사 or 사아 in some dialects).

Can the present tense 사요 also refer to a future event, like “I will buy”?

Yes. Korean often uses the present tense for habitual actions and planned future events.

Habitual: “I buy fruit every Saturday.”
Near-future plan: “I will buy fruit this Saturday.”
Context and time markers clarify the intended meaning.

Is it possible to change the word order of the sentence?

Korean is relatively flexible as long as the verb stays at the end. You could say:

과일을 토요일에 시장에서 사요. (emphasizes 과일을)
시장에서는 토요일에 과일을 사요. (emphasizes the market)
All versions remain grammatical; particles tell you each word’s role.

Why don’t we add the plural marker 들 to 과일 to mean “fruits”?

Korean often treats things like 과일 as mass nouns, so you don’t need 들. If you did say 과일들, it wouldn’t be wrong, but it sounds redundant or overly specific. Context usually tells you whether it’s one piece of fruit or many.