"他去年离开了伦敦。"

Translation: He left London last year.

[Wallace863554]

How common is "去年"??? Could "上个年" be used here?

[keyongming]

去年 is very common. Though 上年 is a word, I don't recall the last time I actually heard someone use it in everyday speech. 上 as a relative time word much more commonly applies to months (上个月) and weeks (上个星期) and, of course, occurs in the word 上午 'A.M.'.

[JoneLiu]

Why isn't "Last year he left London" correct?

[keyongming]

It's fine. Did you report that your answer should have been accepted?

[ASZ18101849]

It is accepted now.

[Ilya57484]

Next time he should take the plane.

[domsalazar0]

How do you know that it's saying "left FROM London" vs "left TO London"?
Got this wrong, so I'm trying to get it straight.

[katja.maucec]

离开 is always used when leaving a place, not leaving towards it. If you wanted to say something along the lines of "left home TO London" you could use 到 or 去, as in "离开家(到/去)伦敦"