が

が at the end of a clause means "but": it joins two clauses that pull against each other. It goes after the plain or the polite form, and it is the formal "but", for writing and careful speech.

A noun or a な-adjective needs だ or です before が. An い-adjective takes が directly.

• × 彼女は学生が、仕事もしています → 彼女は学生だが、仕事もしています
• × 忙しいだが → 忙しいが

が or けど

けど goes in the same place and means the same. けど is what you hear in conversation; が is what you read, and what you say in a meeting or a speech.

Leading into a request

が doesn't always contrast. After すみません or しつれい失礼です, it sets up a request or a question, and English uses "but" or nothing at all.