Learn Hardcore Korean: taeksi daesin beoseureul tayo. - I take the bus instead of a taxi.

In X 대신 Y, you are choosing Y instead of X.
So in 택시 대신 버스를 타요, you choose the bus instead of the taxi.
Flip it and you change the meaning: 버스 대신 택시를 타요 = choose the taxi instead of the bus.

Do I need the particle -에 after 대신? Is 택시 대신에 버스를 타요 also correct?

Yes, both are correct: 택시 대신 버스를 타요 and 택시 대신에 버스를 타요.

대신 and 대신에 are both widely used with minimal difference; 대신에 can sound a touch more explicit or careful in writing, but in everyday speech they’re interchangeable here.

Does 대신 also mean “on behalf of”? How do I tell that apart from “instead of”?

Yes. 대신 has two common meanings:

Substitution of one thing for another: 택시 대신 버스를 타요 (bus instead of taxi).
Doing something on someone’s behalf: 부장님 대신(해서) 발표했어요 (I presented on behalf of the manager).
Context decides the meaning. With inanimate options like vehicles, it’s naturally the “instead of” reading. Adding -해서 after 대신 often signals the “on behalf of” meaning.

Why is there no subject in this sentence?

Korean often drops the subject when it’s obvious from context. The understood subject is usually I/we here. You can add one for clarity or emphasis:

저는 택시 대신 버스를 타요 (As for me, I take the bus instead of a taxi).
우리는 택시 대신 버스를 타요 (We take…).

Why is it 버스를 (object marker) and not 버스가 (subject marker)?

Because 타다 is a transitive verb that takes the vehicle as its direct object: N+을/를 타다 = ride/take N.
So you say 버스를 타요, not 버스가 타요. The subject (often omitted) is the person riding.

Is 타다 “to ride,” “to get on,” or “to take”? When do I use 버스를 타다 vs 버스에 타다?

버스를 타다: to ride/take the bus (general use; very common).
버스에 타다: to get on/board the bus (focus on the boarding action).
Getting off: 버스에서 내리다 (from the bus).
All are natural; choose based on whether you’re emphasizing the ride or the act of boarding.

Can I use 말고 instead of 대신?

Yes: 택시 말고 버스를 타요 is very natural and slightly more colloquial.
Similar option: 택시가 아니라 버스를 타요 (“not a taxi but a bus”), which explicitly negates taxi.
Nuance:

대신 = substitution/alternative.
말고 = “not X but …” in casual speech.
아니라 = clear logical negation of a noun.

Can I say 버스를 대신 타요 without stating what it’s instead of?

Yes. 버스를 대신 타요 means “I’ll take the bus instead,” with the “instead of what” understood from context. Without context, it’s vague but still grammatical.

Can I change the word order, like 버스를 택시 대신 타요, or put 택시 대신 after the verb?

Most natural: 택시 대신 (에) 버스를 타요 (keep X 대신(에) Y together).
버스를 택시 대신 타요: possible but less natural.
Post-verbal: 버스를 타요, 택시 대신(에) can occur in conversation for afterthought emphasis, but it’s not a default word order in writing.

What tense is 타요? Can it also mean future?

타요 is non-past and can mean:

Present/habitual: “I (usually) take…”
Near future/plan: “I’ll take…” (common in context).
To make future explicit: 탈 거예요.
Past: 탔어요.

What politeness level is 타요? How would I say this more formally or as a suggestion?

타요: polite informal (most everyday interactions).
탑니다: formal polite (announcements, formal writing).
Suggestion/request: 버스를 타세요 (honorific imperative).
If you’re the one deciding/volunteering: 버스를 탈게요 (“I’ll take the bus (then)”).

Can I drop the object particle -를 in casual speech?

Yes, in casual conversation particles are often dropped if meaning is clear: 택시 대신 버스 타요.
In careful speech/writing, keep 버스를.

How do I say “Instead of taking a taxi, I walk” (verb-to-verb “instead of”)?

Use -는 대신 for verbs:

택시를 타는 대신 걸어요 (Instead of taking a taxi, I walk).
Or negate the first action:
택시를 타지 않고 걸어요 (I don’t take a taxi; I walk).

Any quick pronunciation and spacing tips?

Spacing: write 택시 대신 버스를 타요 (don’t attach 택시대신).
Revised romanization: Taeksi daesin beoseureul tayo.
Syllables: 택-시 / 대-신 / 버-스-를 / 타-요.
No special linking issues here; just keep a smooth rhythm.