Learn Hardcore Korean: biga omyeon usaneul sseoya haeyo. - If it rains, you have to use an umbrella.

The suffix -면 attaches to a verb stem to create a conditional clause, meaning “if…” or “when…”. You take the stem of 오다 (“오-”) and add -면, giving you 오면 (“if it comes,” i.e. “if it rains”).

What is the purpose of -아/어야 해요 in 써야 해요, and how does it convey obligation?

-아/어야 해요 is used to express necessity or obligation, equivalent to “have to” or “must.” You attach -아야 or -어야 to the verb stem and then add 해요. Here, 쓰다 (to use/put on) → 써- + -야 해요 = 써야 해요, meaning “you have to use (an umbrella).”

What is the difference between 해야 해요 and 해야 돼요?

Both mean “have to do.”

해야 해요 comes from 하다 + necessity (“to have to”).
해야 돼요 comes from 하다 + 되다 (“to become necessary”).
In everyday speech they are almost interchangeable, though some speakers feel -해요 is slightly more direct and -돼요 a bit softer.

Why is 쓰다 used for 우산을 쓰다, and doesn’t 쓰다 also mean “to write”?

쓰다 is a multi-use verb. Depending on context, it can mean:

“to write” (펜을 쓰다)
“to wear/use” for hats, glasses, umbrellas, etc. (모자를 쓰다, 안경을 쓰다, 우산을 쓰다)
Here, because the object is 우산, 쓰다 means “to hold over your head/use” the umbrella.

Can I use 비가 올 때 instead of 비가 오면, and what is the nuance between -면 and -때?

Yes, 비가 올 때 우산을 써야 해요 is grammatically correct. The nuance:

-면 = “if/when,” implying a conditional or hypothetical (“if it rains, then…”).
-때 = “when,” focusing on the time something happens (“at the time it rains…”).
In practice the meaning is very similar here.

Why is it 비가 and not 비는 오면? When do you use -가 versus -는?

-가 marks the subject, introducing new or specific information: “rain comes.” -는 marks the topic or generalizes: “as for rain…” You use 비가 오면 when stating a condition. If you said 비는 오면, it would sound like you’re contrasting rain with something else.

Why do we say 우산을 with -을? What role does that particle play?

-을 is the object marker for verbs ending in a consonant (like 쓰다’s object 우산). It tells us 우산 is the direct object of 쓰다. Without it, the sentence would be ambiguous.

Is it possible to drop particles or shorten this sentence in casual spoken Korean?

Yes. In casual speech you might omit some particles and shorten verb endings:

비 오면 우산 써야 돼
비 오면 우산 써야 해
Dropping 가 and 을 is common in fast, colloquial conversation.

How would I change the politeness level of this sentence to informal or more formal styles?

Informal (friends or close juniors):

비 오면 우산 써야 해. or 비 오면 우산 써야 돼.

Formal polite (business, strangers):

비가 오면 우산을 써야 합니다.

Honorific/formal (very polite):

비가 오면 우산을 쓰셔야 합니다. (adds -시-)