When something is off the charts — the best, the worst, the most you've ever seen — tack 极了 (jíle) onto the adjective. 极 means "the furthest point, the extreme" (it's the 极 of 北极, the North Pole), so 好极了 reads literally as "good — all the way to the pole." It works for praise and complaints alike, and it lives in casual, spoken Chinese.
Unusually for a degree phrase, 极了 clamps straight onto the adjective with no 得 — and the 了 is welded on: drop it and the sentence is simply wrong.
Structure: Subject + Adjective + 极了
Literally: "[subject is] adjective — to the extreme."
this这个idea主意good好extremely极了。
This idea is perfect.
the college exam高考that day那天I，我nervous紧张extremely极了。
I was incredibly nervous on the day of the gaokao.
that那个boy男孩annoying讨厌extremely极了！
That boy is unbelievably annoying!
Remember: 极 = pole, the furthest point (北极 = North Pole). 好极了 pushes "good" all the way to the pole — and 极 never travels without its 了。
Watch out
- The 了 is non-negotiable: 好极了 ✓, never 好极。
- No 得 — it attaches bare (好极了), unlike most degree complements (好得很)。
- 极了 already carries the emphasis, so don't pile on 很 or 非常: 好极了, never 很好极了。