Learn Hardcore Turkish: O kadar yorgundum ki vazgeçmek üzereydim, ama arkadaşım beni destekledi. - I was so tired that I was about to give up, but my friend supported me.

The correlative pattern o kadar ... ki means “so … that.” Put o kadar before the word you want to intensify, then use ki to introduce the result clause.

Example from your sentence: O kadar yorgundum ki vazgeçmek üzereydim. = “I was so tired that I was about to give up.”
Another example: O kadar hızlı konuştun ki anlamadım. = “You spoke so fast that I didn’t understand.”

Can I replace o kadar ... ki with öyle ... ki? Are they interchangeable?

Often yes, especially with adjectives/adverbs:

Öyle yorgundum ki vazgeçmek üzereydim. = “I was so tired that I was about to give up.” Nuance:
o kadar emphasizes degree/amount and works well with quantities too (e.g., o kadar yemek = that much food).
öyle emphasizes manner/quality and is mostly used before adjectives/adverbs. In your sentence, both are fine; o kadar slightly highlights the degree of tiredness.

What is ki doing here? Is it the same ki as in Dedi ki… or Demek ki…? Can I omit it?

Here ki is the linker in the result construction o kadar/öyle … ki (“so … that …”). It’s not exactly the same use as:

Dedi ki… (“He said that…”) → reported speech.
Demek ki… (“So/therefore…”) → inferential adverb. You generally shouldn’t omit ki if you want the explicit “so… that…” meaning. Without it, you’d need another connector: Çok yorgundum; bu yüzden vazgeçmek üzereydim.

Why is it yorgundum and not yorgunum?

yorgunum = “I am tired” (present).
yorgundum = “I was tired” (past). Form: adjective/noun + past copula + person
yorgun + du + m → yorgundum (vowel harmony picks -du- because of the last vowel u).
Negative: yorgun değildim (“I wasn’t tired”). Other persons: yorgundun (you were), yorgundu (he/she/it was), yorgunduk (we were), etc.

How does vazgeçmek üzereydim work? What is üzere and why does it take -ydim?

Verb-mek/-mak + üzere
- copular ending expresses imminence: “to be about to Verb.”
Present: Gitmek üzereyim (“I’m about to go.”)
Past: Gitmek üzereydim (“I was about to go.”)
3sg past: Gitmek üzereydi (“He/She was about to go.”)
Negative: use değil → Gitmek üzere değildim. What’s -ydim? Historically it’s idi (the past copula). After a vowel, it surfaces as -ydi/-ydı/-ydu/-ydü. So:
üzere + idi + m → üzereydim (the y is a buffer consonant).

What’s the difference between vazgeçmek üzereydim, vazgeçecektim, and neredeyse/az kalsın vazgeçiyordum?

vazgeçmek üzereydim: I was just on the verge of giving up (imminent, seconds away).
vazgeçecektim: I was going to give up (a planned/expected future in the past; not necessarily immediate).
neredeyse/az kalsın vazgeçiyordum: I almost/nearly gave up (focus on how close it came to happening; az kalsın is stronger colloquially).

Does vazgeçmek take a case? What would the object look like?

Yes. vazgeçmek takes the ablative (-den/-dan):

Projeden vazgeçtim. = “I gave up on the project.”
Sigara içmekten vazgeçti. = “He/She gave up smoking.” In your sentence, no object is stated, so it’s just the bare infinitive vazgeçmek.

Why is it beni and not ben in arkadaşım beni destekledi?

Because beni is the accusative form of ben (“me”). desteklemek (“to support”) is a transitive verb, so the direct object takes the accusative:

Arkadaşım beni destekledi. = “My friend supported me.” Pronouns as direct objects are always marked (beni, seni, onu, bizi, sizi, onları).

Could I say Arkadaşım bana destek oldu instead of Arkadaşım beni destekledi? Any nuance?

Yes:

Arkadaşım beni destekledi. = “My friend supported me.” (verb desteklemek; direct, active support)
Arkadaşım bana destek oldu/verdi. = “My friend gave me support.” (noun destek
- light verb; a bit more idiomatic/less direct) Both are natural; the first often sounds more straightforward and active.

Can I change the word order for emphasis?

Yes, Turkish allows flexible word order to shift focus:

Neutral: Ama arkadaşım beni destekledi.
Emphasizing who did it (contrasting with others): Ama beni arkadaşım destekledi.
Emphasizing the object: Ama arkadaşım beni destekledi (default already focuses the verb-object sequence). The verb typically stays at the end of its clause; moving constituents changes emphasis, not core meaning.

Where does ama go, and do I need a comma?

Ama (“but”) often starts the contrasting clause: ..., ama ...
A comma before ama is common and recommended in writing: ..., ama ...
You can also start the sentence with Ama for a contrast with previous context: Ama arkadaşım beni destekledi.

Does arkadaşım automatically mean my friend? When would I use benim or bir?

arkadaşım = “my friend” (possessive -ım). It usually refers to a specific friend already known in context.
benim arkadaşım adds emphasis/contrast: “my friend (as opposed to someone else’s).”
bir arkadaşım = “a friend of mine” (introduces an unspecified one).

What exactly is happening morphologically in yorgundum and üzereydim?

yorgun-du-m: adjective yorgun
- past copula -du- (harmonized) + 1sg -m → “I was tired.”
üzere-ydi-m: postposition üzere
- past copula -ydi- (from idi, with buffer y) + 1sg -m → “I was about to …”

What tense is destekledi, and how is it formed?

destekledi is simple past (di-past), 3rd person singular:

Root: destekle- (“to support”)
Past: -di (harmonizes to -di/-dı/-du/-dü; here -di)
3sg has no personal ending: destekle-di → “(he/she) supported.”

Are there alternatives to ama? Do they change the tone?

Yes:

fakat: “but,” a bit more formal than ama.
ancak: “however/but,” often more formal; also means “only” depending on context.
lakin: very formal/literary “but.” Your sentence with any of these is fine; ama is the most neutral in everyday speech.

Any other natural ways to phrase the first half?

Çok yorgundum; neredeyse vazgeçecektim.
Öyle yorgundum ki az kalsın vazgeçecektim.
Tam vazgeçmek üzereydim ki arkadaşım beni destekledi. (Here ki means “just as … then …,” a different but useful pattern.)