Why use 가 in 의사가 되세요?

Question

I'm learning about the usage on (으)세요, but my textbook explains that you cannot use 이다 in (으세요) form. So the following sentence is invalid:

의사이세요

However, the textbook says the correct form is this:

의사가 되세요

But I don't know why you must use 가 there. Because the sentence means "(You) Become a doctor.", where a doctor is an object, I wonder why the sentence use 를, like:

의사를 되세요

Or 에, to mean the subject is going to.

의사에 되세요

For me, the sentence looks like a doctor is a subject, not an object...

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UPDATE

The example sentence comes from a book Korean Grammar in Use (Beginner), Unit 7, Page 167.

Answer 1

I'm learning about the usage on (으)세요, but my textbook explains that you cannot use 이다 in (으세요) form. So the following sentence is invalid:

의사이세요

You can add the honorific particle (으)시 to 이다 and the 요 ending, to make 이세요. When you add this to a word that ends in a vowel, like 의사, the initial '이' just gets dropped due to spelling/pronunciation conventions, giving you '의사세요'.

Put another way, 의사이세요 is 'logically' fine for 'you are a doctor' - it's just that the spelling should be contracted to 의사세요.

However, the textbook says the correct form is this:

의사가 되세요

As you say, the most straightforward translation of this is 'You become a doctor'. I'm not sure if it might be seen as an idiomatic polite way to say 'you are a doctor' - let's assume not, unless a native speaker corrects me on this point! Edit : based on comments below, 'please become a doctor' is a likely interpretation. There is a general ambiguity about the '세요' ending, as it may be used for statements, questions, and imperatives.

The 가 is not a subject marker here, but the complement case marker, as explained in this answer. It's basically a special thing used only with 되다 and 아니다, so you are right to notice that it's a bit strange!

Answer 2

After reading your question and latest comments again, now I can see where your confusion comes from. Actually, you seem to have the same difficulties that I had when I learned Japanese.

You need to note that the particles, endings, and markers in Japanese don't always have the same counterparts in Korean. In other words, they differ depending on a sentence structure. For example,

Please become a doctor.

could be translated to

의사가 되세요. or 의사가 되어주세요.

"医者になってください" in Japanese means the latter, not the former as 'ください' means '주세요'. Just because the Japanese sentence use 'に' after '医者' doesn't necessarily mean the Korean sentence should be

*의사에 되세요.

This sentence doesn't make any sense. Neither does "의사를 되어 주세요". I know 'に' is mostly translated to '에' in Korean, but in this case, no.

It would be more helpful if you try to remember such example sentences as

착한 사람이 되세요. 착한 사람이 되어 주세요. Please be a good person.

You need to note that you should use '이' when the preceding block has a final consonant (받침) such as '사람'.

Answer 3

(Expanding a bit on what Mallory-Erik said in the comments to topo morto's answer...)

Sometimes "되다" can be used as a polite expression instead of "-이다". However, I think the "complement case marker" -이/가 is usually dropped in this case. For example:

의사 되세요? (= 의사세요?) Are you a doctor?

이분이 아까 전화하신 분 되세요? (= 이분이 아까 전화하신 분이세요?) Is he/she the person who called (us) before?

제 선생님 되십니다. (= 제 선생님입니다.) He/she is my teacher.

In each example, the first sentence is perceived as "more polite" to the person referred to (의사/이분/선생님).

So "의사(가) 되세요" could mean "Are you a doctor?" or "He/she is a doctor." depending on intonation.

If taken as imperative, "의사가 되세요" could also mean "Please be a doctor", but I think it's an unlikely sentence in everyday situation: as always, it's a matter of context. (Here, using -가 is more natural: using "의사 되세요" as an imperative is not impossible, but it would sound even more unnatural.)