Learn Hardcore Korean: naeil yakguge gaseo piryohan yageul sayo. - I will go to the pharmacy tomorrow to buy the necessary medicine.

에 marks the destination of the motion verb 가다 (to go). In 약국에 가서, it tells you “go to the pharmacy and then….” Without 에, you wouldn’t have a clear “to” or “towards” meaning. (Contrast with 약국에서 사요, where 에서 marks the location “at the pharmacy,” not the destination.)

What’s the function of the connective ending –아서 in 가서? How would the meaning differ if we used –고 instead?

–아서/–어서 attaches to verb stems to show sequence (“and then”) or cause (“because”). Here, 가서 means “go and then…buy.” If you used 가고, you’d still connect two actions (“go and buy”), but –고 is a more neutral connector. In practice both work for simple sequences, though 가서 can feel slightly more natural when the first action directly enables the second.

Why is 필요한 used before 약 instead of just 필요하다?

필요하다 is a descriptive verb (“to be necessary”). To turn it into an adjective modifying a noun, you use its attributive form 필요한. So 필요한 약 literally means “the medicine that is needed.” You can’t place 필요하다 directly in front of 약; you need 필요한 to form the modifier.

Why is the object marker 을 used in 약을 사요?

을 (after consonants; 를 after vowels) marks the direct object of a verb. Since 약 (“medicine”) is what you’re buying, it takes 을. Without it, the sentence would lack a clear object and feel incomplete.

What level of politeness is 사요? Could we use a different ending?

사요 is the polite informal form (the –아/어요체). It’s standard for everyday conversation with people you don’t know well or with whom you want to be polite. Alternatives:
• 삽니다 (formal polite, –ㅂ니다체)
• 산다 (plain/neutral, often in writing or diaries)
• 살 거예요 (polite future) if you want to emphasize “will buy.”

Why is the subject omitted in this sentence? How do we know who is doing the action?

Korean frequently drops subjects when they’re obvious from context. Here, because you start with 내일 (“tomorrow”) and you’re describing your own plans, it’s clear that the speaker is the one going and buying. If you needed clarity, you could add 제가 (“I”) or 나는 before the verb phrase.

How can we express the future more explicitly, for example using –겠다 or –ㄹ 거예요?

You can attach future markers to the verb stems:
• 갈 거예요 / 살 거예요 → 내일 약국에 가서 필요한 약을 살 거예요 (“Tomorrow I will go to the pharmacy and buy the medicine”).
• –겠다 (stronger/fixed intention) → 내일 약국에 가서 필요한 약을 사겠다, which sounds more formal or like a vow.

What’s the difference between 약국에 가서 약을 사요 and 약국에서 약을 사요?

• 약국에 가서 약을 사요: emphasizes the two-step action—first “go to the pharmacy,” then “buy medicine.”
• 약국에서 약을 사요: simply states “buy medicine at the pharmacy,” focusing on the location of the buying, not the act of going there first.