「彼{かれ}ら は 自分{じぶん} の 事{こと} しか 考{かんが}えて いません。」
While the translation:
"They only think of themselves."
is a valid one in the sense that it successfully conveys the basic meaning of the original, it can also be highly misleading as far as the grammatical understanding of the original.
The original Japanese sentence is indeed in the negative form even though the English translation I just called 'valid' is clearly in the affirmative form.
The key word is 「しか」, which is always used in conjunction with a negative verb phrase.
「Noun + しか + Verb in Negative Form (Verb + ない/ません, etc.)」
Literally means:
"to not [Verb] anything but [Noun]"
Therefore,
「自分のことしか考えていません」
literally means:
"not thinking about anything except for themselves"
which in turn means:
"to only think about themselves"
The less wordy translations, therefore, tend to be used more commonly over time.
[NEG VERB]not translate well into English. – Eiríkr Útlendi Nov 21 '19 at 18:10Just as the "a / the" distinction, grammatical number, or gender cannot be translated well into Japanese, so too do things like しか...[NEG VERB] not translate well into EnglishI disagree. I think しか〜ません translates perfectly into English. 彼らは ("They"), 考えていません ("do not think (of)"), 自分の事しか ("(anything) but themselves"). – istrasci Nov 21 '19 at 18:25[NEG VERB]I should have added "not always translate well". (Some things just flat don't translate, like the は・が distinction, contrastive は, subtleties in semantic transitivity in 他動詞 vs. syntactic transitivity in English "transitives", etc.). – Eiríkr Útlendi Nov 21 '19 at 18:51