YOU SAID:
Ele veio, ai ele não conseguir
INTO JAPANESE
彼は来ました、そして彼はできませんでした
BACK INTO ENGLISH
he came and he couldn't
INTO JAPANESE
彼は来ましたが、できませんでした
BACK INTO ENGLISH
he came but he couldn't
INTO JAPANESE
彼は来ましたが、できませんでした
BACK INTO ENGLISH
he came but he couldn't
That didn't even make that much sense in English.