去年

English loves putting 'last year' at the end: I bought a computer last year. Chinese does not usually do that. Put 去年 at the front, or right after the subject and before the verb. So 去年我买了新电脑 and 我去年买了新电脑 both sound natural. 我买了新电脑去年 sounds like an afterthought and breaks the normal flow. “去年买了新电脑。” (Last year I bought a new computer.) ── [《时间和时候（1）卖车看电影》](/read/time-versus-moment#s15)

English says 'in last year', so learners often add 在. But bare 去年 already means 'last year' and needs no helper. 去年我买了新电脑 is the everyday pattern. 在去年我买了新电脑 sounds stiff or redundant. If you add a specific season, 在去年夏天 is fine because 在 attaches to the longer phrase.

去年 can modify a noun, but general nouns usually need 的 in between. 去年的照片 means photos from last year. 去年照片 sounds like a hurried label on a folder. Fixed time phrases like 去年夏天 and 去年十二月 often skip 的. For normal things like 计划 or 照片, keep 的.

去年 already tells you the time is last year. But 了 still does a different job: it marks the buying as done. Without 了, 去年买新电脑 sounds like a fragment, not a finished sentence. So 去年买了新电脑 is the natural full sentence. The 了 sticks to 买, not to 去年.