从句不一定要陈述语序吗？也可以是疑问句吗？像下面这句话

Question

Our correspondent says it is too early to say what impact this will have on
the ceasefire across the eastern region, but it doesn't bode well

Answer 1

这句里的从句不正是陈述语序吗？
what impact this will have...

如果是疑问句，what impact will this have?

从句里不一定都是陈述语序，比如在宾语从句中引导词作主语的情况，例如，
I don't know what is wrong with him. what在从句中作主语，所以后面语序不用改为陈述。追问

那这句话中what impact为什么要放在前面？因为主语是this ？不能放在this will have后面吗？

追答

因为问的是[什么影响]，而不是[什么]。例如which word do you have in mind? 问的是[哪个单词]。

疑问句what impact will this have on the ceasefire？ 这会对停火协定造成什么影响？
从句则是 what impact this will have on the ceasefire. 陈述语序

陈述句This will have（bad) impact on the ceasefire. 括号里是疑问句问的东西。

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Answer 2

不可以是疑问句
一定要陈述句what impact this will have on
the ceasefire across the eastern region是陈述句

Answer 3

……这个what不是疑问词，它是代词。它是“ impact this"的主语。