biga omyeon ubireul ibeo.

-면 is a conditional ending. It means if or when.

• 오다 = to come
• 오면 = if it comes / when it comes

With weather expressions like 비가 오면, English often translates it as if it rains or when it rains. In this sentence, it gives the condition for the action in the second part: if/when it rains, wear a raincoat.

The particle 가 marks 비 as the subject of the verb 오다 in the weather expression 비가 오다, which literally means rain comes and naturally means it rains.

So:

• 비가 오다 = to rain
• 비가 오면 = if/when it rains

In casual speech, Koreans sometimes drop particles, so you may hear 비 오면 too. But 비가 오면 is the full, standard form.

Korean often describes weather as something that comes.

So:

• 비가 오다 = rain comes = it rains
• 눈이 오다 = snow comes = it snows

This is just the normal Korean way to express rain and snow. Even though it sounds different from English, it is a very basic and common pattern.

It comes from the verb stem plus -면.

• Dictionary form: 오다
• Stem: 오-
• Add -면
• Result: 오면

So 오면 means if/when it comes. In this sentence, because 비가 오다 means to rain, 비가 오면 means if/when it rains.

를 marks 우비 as the object of the verb 입다.

• 우비 = raincoat
• 우비를 = raincoat + object marker
• 입다 = to wear / to put on clothing

So 우비를 입어 means wear a raincoat or put on a raincoat.

Because a 우비 is something you wear.

In Korean:

• 입다 is used for clothes you put on your body
• 쓰다 is used for things like hats, helmets, glasses, or an umbrella in the sense of holding/opening over yourself? Actually for umbrellas, Korean usually uses 쓰다 too

Examples:

• 코트를 입다 = wear a coat
• 우비를 입다 = wear a raincoat
• 모자를 쓰다 = wear a hat
• 우산을 쓰다 = use/wear an umbrella

So 우비 naturally goes with 입다.

By itself, 입어 can be understood in more than one way depending on context, but in this sentence it is most naturally read as a casual command or casual advice:

• 비가 오면 우비를 입어. = If it rains, wear a raincoat.

This is informal speech, used with someone close to you, younger than you, or in a casual setting.

A more polite version would be:

• 비가 오면 우비를 입어요.

So yes, here 입어 works like wear one or put one on.

The final form 입어 is casual, non-polite speech.

You would use it with:

• close friends
• younger siblings
• children
• people you speak casually with

A polite version is:

• 비가 오면 우비를 입어요.

A stronger, more formal command could be:

• 비가 오면 우비를 입으세요.

So the original sentence is natural, but it depends on who you are talking to.

It can mean either, depending on context.

In Korean, -면 often covers both ideas:

• if it rains
• when it rains

In a sentence like this, it often has a general, practical meaning: whenever it rains, wear a raincoat.

So Korean does not always force the same distinction that English does.

Yes. Korean often leaves out subjects when they are obvious from context.

So the full idea might be:

• 네가 비가 오면 우비를 입어.
• You, if it rains, wear a raincoat.

But saying 네가 would usually be unnecessary unless you want emphasis or contrast. Korean prefers to omit pronouns when the listener can already tell who is meant.

Grammatically, 입어 can sometimes look like a casual present-tense form, so without context, Korean forms like this can occasionally be ambiguous.

However, in this exact sentence, the most natural reading is a casual instruction or advice:

• If it rains, wear a raincoat.

If you wanted to clearly say a habitual statement such as I wear a raincoat when it rains, you would usually include context or a subject, for example:

• 나는 비가 오면 우비를 입어.

That makes the statement reading much clearer.