Learn Turkish: Keşke iyi bir kariyer için o diplomayı alsaydım. - I wish I had gotten that diploma for a good career.

It combines the verb root al (to get or take) with the conditional suffix -sa, the past tense suffix -ydı, and the personal ending -m. When used with keşke, this -saydı ending is the standard way to talk about past regrets, meaning you didn't get the diploma but wish you had.

Could I leave out keşke and just say o diplomayı alsaydım?

Yes, the grammar of the verb alsaydım already carries the meaning of "if only I had gotten". However, adding keşke makes the feeling of regret much stronger and clearer right from the start of the sentence.

Why does diplomayı have the -yı ending here?

Because of the word o (that). You are talking about getting a specific diploma, not just any diploma in general. This makes it a specific direct object, so you must add the accusative case ending -yı, using y as a buffer letter since diploma ends in a vowel.

Why is the word order iyi bir kariyer instead of bir iyi kariyer?

In Turkish, when you use an adjective and the indefinite article bir together to describe a noun, the adjective almost always comes first. So "a good career" translates literally to "good a career".