Subjects are required in English finite clauses
Complete sentences in English are made up of at least one “finite clause.”
A finite clause always contains a “finite verb”—that is, a verb that has a changed form showing the grammatical information of tense, aspect, voice, and grammatical mood.
This finite verb also always corresponds to a grammatical “subject.” This subject is either the actor that acts out the action of the finite verb (when the verb is in active voice) or the direct recipient of the action (when the verb is in passive voice).
(ⓘ We refer to “finite clauses” just as “clauses” in our course, YouTube videos, and blog articles to simplify terms.)
English grammar requires that finite clauses have a grammatical subject corresponding to the finite verb. For example:
✓ Now, the baby is eating.
X Now, ^ is eating.
“Now, the baby is eating” is one finite clause. Its finite verb is “is eating,” a form that shows present tense, progressive aspect, active voice, and indicative mood.
In English grammar, a finite verb requires a grammatical subject. Here, the subject is “the baby.” This subject corresponds to the finite verb in form.
The example above of “Now, ^ is eating” violates this systematic grammatical requirement for grammatical subjects as the finite verb “is eating” has no subject. “Now” is an adverb modifier in the sentence and is not a subject.
As grammar is systematic, this subject requirement applies to all finite clauses in English, even when they are linked to or embedded in other clauses.
For example:
✓ Now, the baby is eating, and the woman is sleeping.
X Now, the baby is eating, and ^ is sleeping.
The above sentences are made up of two finite clauses. The conjunction “and” links the finite clause “the woman is sleeping” to the finite clause “the baby is eating.” This linked clause also requires a subject for its finite verb. The subject here is “the woman,” and the finite verb is “is sleeping.”
✓ I think that the baby is eating.
X I think that ^ is eating.
The sentences above are also made up of two finite clauses. The finite clause “that the baby is eating” is embedded within the outer finite clause of “I think [……]”
Even when a finite clause is embedded in another clause, the systematic requirement for subjects still applies. The finite verb “is eating” still requires the subject “the baby.” The finite verb of the embedded clause, “is eating,” cannot have no subject.
(ⓘ Even when seeming “exceptions” to the subject requirement arise, we must understand them within the same framework of systematic grammar.)
On the other hand, Cantonese grammar (and the grammars of other languages in the Chinese family) allows the subject of a sentence to be dropped when the meaning is clear in context. Because of this, many Cantonese speakers instinctively drop the subject of an English finite clause as well, forming sentences that violate a systematic rule of English grammar.
For example, when native Cantonese speakers wish to express “我覺得佢無事,” Cantonese grammar allows the subject of “無事”, i.e. “佢,” to be dropped if it is clear in context. That is, “我覺得（佢）無事” is a grammatically correct sentence.
This leads them to say or write a sentence in English such as:
X I think that ^ is okay.
However, English grammar requires a subject and does not allow a subject to be dropped. That is why this sentence, whose embedded clause “that ^ is okay*” has no subject, is grammatically incorrect in English. The subject of the embedded clause must be added back in for this sentence to be correct. For example:
✓ I think that HE is okay.
Understanding this systematic “subject requirement” of English grammar allows you to check your own tendency to drop the subject when producing English sentences and correct the structural mistakes you are prone to make in a more systematic way.
(ⓘ The blog section on the grammatical differences between Cantonese and English has more examples of the mistakes that native Cantonese speakers would instinctively make if they are unaware of English systematic grammar.)
「subject」（主語）在英文限定子句中是必需的
英文的完整句子是由至少一個「限定子句」（finite clause）所組成。
一個限定子句中包含了一個「限定動詞」(finite verb)，即會轉變字形去顯示時態、體貌、語態和文法語氣等文法資訊的動詞。
這個限定動詞必定要跟一個「主語」對應。這個主語跟對應的限定動詞是有直接的關係的：它要不是作為那限定動詞動作的「實行者」（當動詞為「主動語態」時），便是該動作的「直接接收者」（當動詞為「被動語態」時）。
（ⓘ在我們的正式課程、YouTube視頻和博客文章中，我們將「限定子句」簡稱為「子句」。）
英文文法要求限定子句必需包含一個對應限定動詞的文法主語，例如:
✓ Now, the baby is eating.
X Now, ^ is eating.
「Now, the baby is eating」是限定子句。其限定動詞是「is eating」，字形顯示出現在式時態、進行式體貌、主動語態，以及陳述語氣。
英文文法中，要求限定動詞需要配合一個文法主語，正如這裡的主語「the bady」與便限定動詞字形相呼應，是那限定動詞「is eating」的「實行者」。
至於另一個例句「Now, ^ is eating*」則沒有滿足這系統性文法規則的要求，因為限定動詞「is eating」沒有對應的主語。「Now」只是這句中的一個表達時間意思的副詞修飾語。
由於文法是有系統的，這個對主語的要求會規範英文中的所有限定子句。即使子句是被連接（linked） 或被嵌入 （embedded）到其他子句中，它們也一定需要包含一個主語。
例如:
✓ Now, the baby is eating, and the woman is sleeping.
X Now, the baby is eating, and ^ is sleeping.
上述兩句皆由兩個限定子句組成。限定子句「the woman is sleeping」被連接詞「and」連表到另一個限定子句「the baby is eating」。這個被連接的子句也同樣需要一個主語，而這裡的主語是「the woman」，限定動詞是「is sleeping」。
✓ I think that the baby is eating.
X I think that ^ is eating.
上述兩句也同樣是由兩個限定子句組成。限定子句 [ that the baby is eating ]被嵌入在外層的限定子句 [ I think [……] ] 之中。
雖然這裡，有一個限定子句被嵌入在另一個外層子句中，但英文文法對子句的主語這系統性的要求仍然會規範這子句，所以限定動詞「is eating」便需要有主語「the baby」配合。
（ⓘ就算當這個對主語的系統性要求有時出現似乎是「例外」的情況，我們也需要把這些情況放在整個系統性文法框架中理解。)
廣東話（及其他中文語系語言）的文法卻容許省略句子中的主語，只要有足夠清晰的前文後理去表達意思便可。這也導致不少操廣東話的人士在使用英文時也本能地省略了限定子句中的主語，因而組合出一些有違英文文法系統性規則的英文句子。
例如，當母語為廣東話的人士想表達「我覺得佢無事」，因為廣東話文法容許在有足夠清晰的前文後理下省略主語，所以即使他說的是「我覺得（佢）無事」，省略了「無事」的主語「佢」，文法也是正確的。
如果他按廣東話的文法邏輯來把這句子換成英文，而且不理解英文子句需要主語的系統性規範，便會說:
X I think that ^ is okay.
英文文法並不允許子句的主語被省略，因此這個沒有主語的嵌入式子句「that ^ is okay*」便是一個在英文文法中的系統性錯誤。要令這子句正確，我們要加入主語，例如：
✓ I think that HE is okay.
明白主語在英文子句中的必要性，你可以檢視自己在寫英文句子時會否也有省略主語的傾向，並能以更有系統的方式糾正你容易犯的結構性錯誤。
（ⓘ我們博客中的「廣東話 VS. 英文」部分會更詳細地探討中文和英文的文法差異和母語是中文的人如果對英文系統性文法沒有理解便會很自然犯的錯誤。）