אִם־כֵּנִ֣ים אַתֶּ֔ם אֲחִיכֶ֣ם אֶחָ֔ד יֵאָסֵ֖ר בְּבֵ֣ית מִשְׁמַרְכֶ֑ם וְאַתֶּם֙ לְכ֣וּ הָבִ֔יאוּ שֶׁ֖בֶר רַעֲב֥וֹן בָּתֵּיכֶֽם׃
Si sois hombres de verdad, quede preso en la casa de vuestra cárcel uno de vuestros hermanos; y vosotros id, llevad el alimento para el hambre de vuestra casa:
Or HaChaim
It may also be that the brothers did not even consider being separated. The only reason that they eventually agreed to leave Shimon behind was that Joseph had made this a test of how they could establish their credibility. I shall explain this in greater detail in my commentary on verse 19.
Ramban
AND HE PUT THEM ALL TOGETHER INTO A WARD THREE DAYS. This he did in order to frighten them and to make them believe that it is G-d he fears,149Verse 18 here. meaning that it is because of his fear of G-d that he is releasing them lest the people of their households perish from hunger. This is the meaning of his words, The rest of you, go and take grain home to your starving households.150Verse 19 here. Besides, they would not have consented to leave the one [Simeon]151See Verse 24. behind except for the purpose of saving all their families.
Or HaChaim
ויקה מאתם את שמעון, He took Shimon from them. The brothers had singled out Shimon, or Shimon had volunteered to be the hostage as I have explained on verse 19.