Learn Hardcore Turkish: Ben otobüse biniyorum, oysa arkadaşım yürüyerek gidiyor. - I am taking the bus, whereas my friend is walking.

otobüs-e biniyorum = I’m getting on the bus.
Using accusative (otobüsü) here is ungrammatical.
otobüs-te means “on the bus” (location), e.g., Otobüste uyuyorum = I sleep on the bus.

What exactly does oysa mean, and how is it different from ama or halbuki?

oysa means “whereas/while” and sets up a contrast between two clauses. It’s a bit more formal/literary than ama (but). halbuki also means “whereas/however,” often with a “contrary to what one might expect” feel. All three work here:

Ben otobüse biniyorum, ama arkadaşım yürüyerek gidiyor. (more neutral)
Ben otobüse biniyorum, oysa arkadaşım yürüyerek gidiyor. (more formal/contrastive)
Ben otobüse biniyorum, halbuki arkadaşım yürüyerek gidiyor. (counter-expectation)

Could I use oysa ki instead of oysa?

Yes. oysa ki is a slightly more emphatic/flowing variant. Meaning and usage are essentially the same in this sentence.

Can I drop Ben at the start?

Yes. Turkish is pro-drop. Otobüse biniyorum, oysa arkadaşım yürüyerek gidiyor is fully grammatical. Keeping Ben adds emphasis/contrast with arkadaşım.

Why is it yürüyerek gidiyor instead of just yürüyor?

yürüyerek uses the converb -erek/-arak to express manner: “by walking.” So yürüyerek gidiyor means “goes by walking (on foot).”

Arkadaşım yürüyor = My friend is walking (may not imply going somewhere).
Arkadaşım yürüyerek gidiyor = My friend goes (to a place) on foot. A concise synonym is yaya gidiyor (“goes on foot”).

What is the -erek in yürüyerek?

It’s the converb meaning “by doing/while doing,” formed as:

Verb stem + -erek/-arak (vowel harmony).
If the stem ends in a vowel, a buffer -y- appears: yürü- + -erek → yürüyerek. It commonly modifies another verb by giving manner or simultaneity: gülerek konuşmak (to speak while smiling).

Isn’t yürüyerek gidiyor redundant? Why use both?

It’s idiomatic, not redundant. gitmek supplies the motion “to go,” and yürüyerek supplies the manner “by walking.” Together they neatly express “go on foot.”

What tense/aspect is biniyorum / gidiyor? Can it also mean a habitual action?

The suffix -iyor is the present continuous. Depending on context, it can also describe near-future or regular/habitual actions:

Right now: “I’m getting on the bus (now).”
Habitual with adverbs/context: “I take the bus (these days).”

Why is it arkadaşım and not benim arkadaşım?

The possessive -ım/-im/-um/-üm on arkadaş already means “my friend.” Benim arkadaşım is also correct but adds emphasis/contrast (“my friend (as opposed to someone else)”).

How do we know whether arkadaşım is he or she?

Turkish doesn’t mark gender in nouns or third-person verbs. Arkadaşım can be “he” or “she.” Context provides gender if needed.

Could I use ise instead of oysa to mark the contrast?

Yes: Ben otobüse biniyorum; arkadaşım ise yürüyerek gidiyor. ise after a noun phrase marks a contrastive topic (“as for…” / “whereas”). It’s a very natural alternative.

What about word order—can I say Arkadaşım gidiyor yürüyerek?

Best keep the manner adverbial before the verb: Arkadaşım yürüyerek gidiyor. Post-verbal yürüyerek sounds odd in neutral speech. Turkish prefers [Subject] [Manner/Place/Time] [Verb] order.

Why is there a comma before oysa?

You’re linking two independent clauses with a contrastive connector. A comma (or a semicolon) before oysa is standard punctuation.

How would I say “get off the bus”?

Use inmek, which takes the ablative (-den/-dan): otobüs-ten inmek.

Otobüse binmek (dative: onto) vs otobüsten inmek (ablative: off).

What’s the difference between otobüse biniyorum and otobüsle gidiyorum?

otobüse biniyorum = I’m getting on the bus (focus on the act of boarding).
otobüsle gidiyorum = I go by bus (focus on means of transport). Both can be true in the same situation but highlight different aspects.

Why is it otobüse with -e, not -a?

Dative -e/-a follows two-way vowel harmony: after front vowels (e, i, ö, ü), use -e; after back vowels (a, ı, o, u), use -a. otobüs has front vowels (o is back, but crucially ö/ü are front; the last vowel is ü), so otobüs-e. Similarly, yürü- + -erek → yürüyerek respects front vowel harmony.