けど details

write + but

kaku kedo

but; though — the spoken-default connective

ちょっと高たかいけど、買かっちゃった。

chotto takai kedo, katchatta.

It was kind of pricey, but I bought it anyway. casual

来月らいげつの分ぶんを予約よやくしたいんですけど。

raigetsu no bun o yoyaku shitai n desu kedo.

I'd like to make a reservation for next month... neutral

Examples

but / though — joins two clauses that pull in different directions

ちょっと高たかいけど、買かっちゃった。

chotto takai kedo, katchatta.

It was kind of pricey, but I bought it anyway. casual

味あじはいいけど、量りょうが少すくないですね。

aji wa ii kedo, ryou ga sukunai desu ne.

The food's good, but the portions are small. neutral

soft preamble or trailing hedge — no real contrast, けど just cushions what follows or leaves the rest unsaid

来月らいげつの分ぶんを予約よやくしたいんですけど。

raigetsu no bun o yoyaku shitai n desu kedo.

I'd like to make a reservation for next month... neutral

行いけたら行いくけど。

iketara iku kedo.

I'll come if I can, but no promises. casual

けれども — the full form — polite speech and careful writing; also stands alone as a sentence-initial conjunction.
けれど — between けれども and けど — common in writing and measured speech.
けども — clipped けれども — same register zone as けど with a dialect-leaning, insistent flavor (だけども).

Nouns and na-adjectives need だ before けど (学生だけど). After polite forms けど is fine (ですけど、ますけど) and softer than が.

のに
Both link contrasting clauses, but けど merely juxtaposes (高いけど買う notes the price), while のに protests a violated expectation and often reads as complaint (高いのに買う implies the purchase defies sense) — swapping のに in for plain けど adds reproach the speaker may not intend.
が
The same connective "but" split by register — けど is the spoken default and drifts casual, が belongs to polite speech and writing; in a business mail mid-sentence が is the safe pick, in chat けど is.