You want to say whether someone can get through the whole amount of something.
序 · The move
1Take the verb and the amount it has to clear (eat all of, carry all of, finish all of).Is the question about handling the WHOLE quantity, not the manner or speed? If manner, use 得-degree instead.
2Wedge 得 in for can-manage, 不 for can't-manage, then close with 了 (liǎo).Did 了 land last, after 得/不, not before the verb?
3Read it back: 得了 carries through to the end; 不了 leaves a remainder.Could the two be swapped without changing sense? If yes, the 得/不 contrast was lost — fix it.
例 · Examples
1這麼多this much菜，你一個人吃得了can finish (the whole lot)嗎？我吃不了can't finish / can't manage to eat it all。
This much food — can you finish it on your own? I can't.
Sentence 了 is le, reporting that an act finished or a state changed; the 了 in 吃不了 is liǎo, asking whether finishing is possible at all. 吃了 = ate it; 吃不了 = can't manage to eat it.
得-degree rates HOW the act went (跑得快); this point rates WHETHER the full amount can be cleared. No description follows — only 了。
✗ 我不能吃了40個餃子 → ✓ 我吃不了40個餃子 (capacity to finish rides inside the verb as V不了, not on a separate 能)
✗ 你拿得嗎這麼多東西 → ✓ 這麼多東西你拿得了嗎 (the 了 must close the complement before the question 嗎)
✗ 吃了不40個餃子 → ✓ 吃不了40個餃子 (不 sits between verb and 了, never after 了)
English splits 'can' off as a separate modal ('I can't eat them all'); Mandarin folds the can/can't into the verb tail as V得了/V不了, with no standalone 能.