biga omyeon jajeongeo daesin jihacheoreul tayo.

-면 is a conditional ending meaning if or when, depending on context. In this sentence it’s often understood as a habitual when/if it rains, I take the subway instead of a bike.

• 비가 오면 … = If/When it rains, …
  If you want to emphasize a one-time future possibility, if fits well; for a routine, when often sounds more natural in English.

비가 오다 literally means rain comes/falls, so 비 is the subject of 오다, and the subject particle -가 is natural.

• 비가 와요 = It rains
  비는 could be used for contrast or topic-setting (e.g., As for rain, it’s coming…), but the neutral default is 비가.
  비를 is generally not used because 비 isn’t the object of 오다.

They overlap but aren’t identical.

• 비가 오면 focuses on the condition: if/when it rains, then… (cause/condition → result).
• 비가 올 때 means when it rains / at the time it rains and is more purely time-based.
  For routines, both can work, but -면 often feels more like condition → choice/action.

대신 means instead (of) / in place of. The common pattern is:

• A 대신 B(를) 하다/타다/먹다… = do B instead of A
  So 자전거 대신 지하철을 타요 = I take the subway instead of a bike.
  You can also say 자전거를 타는 대신 지하철을 타요 if you want to spell out the verb on the first option (instead of riding a bike, I ride the subway).

In everyday Korean, N + 대신 commonly appears with no particle:

• 커피 대신 차 마셔요 = I drink tea instead of coffee
  You can see 자전거를 대신(으로) in some contexts, but the most natural conversational pattern here is simply 자전거 대신.

With the verb 타다 (to ride / to take), the vehicle is marked as the object with -을/를:

• 버스를 타요, 지하철을 타요, 택시를 타요
  So 지하철을 타요 is the standard way to say I take the subway.

Yes, but the nuance changes slightly.

• 지하철을 타요 = natural, everyday I take/ride the subway
• 지하철을 이용해요 = I use the subway, a bit more formal/neutral (common in announcements, writing, polite explanations)

타요 is 해요체 (polite casual), very common in daily conversation. Alternatives:

• Informal: 타 (to close friends)
• More formal polite: 탑니다 (합니다체)
• Softer/polite suggestion tone (context-dependent): 타는 편이에요 (I tend to take…)

By default, 비가 오면 … 타요 often reads as a habitual choice: Whenever it rains, I take the subway.
If you want to clearly mark a specific future plan, you might add time context or intention:

• 내일 비가 오면 지하철을 탈 거예요 = If it rains tomorrow, I’ll take the subway.

Korean commonly uses 오다 (to come) for weather phenomena:

• 비가 오다 = to rain
• 눈이 오다 = to snow
  It’s just the standard idiomatic pairing in Korean.

Yes. Korean word order is flexible as long as particles/markers make roles clear. For example:

• 비가 오면 지하철을 자전거 대신 타요 (emphasis on subway)
• 비가 오면 대신 지하철을 타요 (works if instead is clear from context, but usually you keep 자전거 대신 together for clarity)