"They have never cooked crab."
Переклад:Вони ніколи не готували краба.
The meaning of the sentence "They have never cooked a crab" depends markedly on where the emphasis is placed in speaking. If a strong emphasis is placed on the word "a," then the implication is that not only have they, in fact, cooked crab but when they have cooked crab, they have always cooked more than one crab. However, this intonation is rare and emphasizes in a manner that frankly sounds deliberately odd / funny that the standard meaning of this sentence is not the one intended.
The more standard meaning is basically the same as that of "They have never cooked crab" only with additional emphasis, something like "They have never cooked even one crab."