will he come back on Sunday?I don't know.（合并为一句）

will he come back on Sunday?I don't know.（合并为一句）
will he come back on Sunday?I don't know.（合并为一句）

will he come back on Sunday?I don't know.（合并为一句）
I don't know if he will come back on Sunday.

I don't know if / whether he will come back on Sunday .
加油！不明白再问！如果帮到你，请采纳，谢谢！