修飾語後面可以加名詞子句嗎?

看看這樣寫是否合你的需要（在書寫時，用冒號帶出同位語，會比較清楚）：

Before going to school, there is one thing to do: you should have a good breakfast.（名詞子句當同位語）
Before going to school, there is one thing to do: having a good breakfast.（適用每個人的忠告，動名詞having與前面的going是相同的詞性，都是動名詞。這是動名詞片語當同位語。句子更佳。）
Before going to school, there is one thing for you to do: having a good breakfast.（加上for you，就限制了適用的對象）

「用」早餐，通常不會用動詞eat，而是have。
如寫成you have breakfast，會有語意不完整的問題（是一件事實嗎？）。加上助動詞should，就表示所述是講話者的一項「主張」或「要求」，句意會較合理。
副詞子句改成片語before going to school，會讓全句結構較簡潔。寫作時，不但求語法正確，也要想辦法讓句子結構漂亮。
one thing to do是一個名詞片語，應視為一個整體。不定詞to do 是「修飾語」，可以再接副詞或副詞子句當其「修飾語」，如副詞quickly。如果接上名詞子句，就跟這個不定詞沒直接關係了，而是跟它所屬的整個名詞片語有關係。

Government Warning:-
you have to eat breakfast=to eat your breakfast
before you go to school=a clause of condition
That you eat your breakfast-----main clause;------Subj.
is the thing to do-----------------infinitive phrase------------VO
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In Grammar THAT is more often left out when the subject of that clause is the same as the subject of the main clause , or where it is a pronoun;
eg:-She knows that she'll eat her breakfast.
eg:-She was told she'd eat her breakfast.
eg:-INFO were that Breakfast was a necessary for sc.success !
eg:-She learnt that breakfast was a necessity for school success=to eat her breakfast.
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THAT is usually put in if main verb is passive, or where the that clause does not immediately follow the verb
eg:-She was told that she should eat her breakfast.
eg:-Government warned her that it was a a school failure if she didn't eat her breakfast HK.

there在本句是當虚（假）主詞，真正主詞是名詞子句that you eat breakfast ，遣種用法是因主詷太長，避免全句頭重腳輕才如此寫法。
one thing to do
其中 不定詞片語to do当形容詞修飾thing,另to do不需接名詞子句，意思已完整。
you eat breakfast 是完整子句

原句沒有you have to eat breakfast(你一定要吃早餐)這意思，我認為你把太多意義讀進去來想句子的結構。

當然假如是媽媽跟兒子說「你上學前必定要吃早餐」:
Before you go to school, you have to eat breakfast.便可以了。

原句意思是表示"上學前會做的一件事是甚麼..."
原句
Before you go to school , there is one thing to do that you eat breakfast.
本身是很累贅的寫法/講法。
而你所指出較順暢的
Before you go to school , you have to do one thing that you eat breakfast.
其實是更累贅，因為再加上you have to這本來沒有的意思(必定/一定要做)。
這句本身文法/句式上是有缺點的，應該是
Before you go to school , you have to do one thing AND that IS THAT you eat breakfast.

還是說名詞子句可不可以改成 that you have to eat breakfast