When something is off the charts — the best, the worst, the most you've ever seen — tack extremely极了 (jíle) onto the adjective. extremely极 means "the furthest point, the extreme" (it's the extremely极 of the North Pole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, extremely极了 clamps straight onto the adjective with no (degree marker)得 — and the (completed / change)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: extremely极 = pole, the furthest point（the North Pole北极 = North Pole）。好极了 pushes "good" all the way to the pole — and extremely极 never travels without its (completed / change)了。
Watch out
- The (completed / change)了 is non-negotiable: 好极了 ✓, never 好极。
- No (degree marker)得 — it attaches bare（好极了）, unlike most degree complements（good好(degree marker)得degree / adjective linker很）。
- extremely极了 already carries the emphasis, so don't pile on degree / adjective linker很 or very非常: 好极了, never degree / adjective linker很好极了。