And, for germs and bacteria that enter the body when a person eats food, there are acids in the stomach that kill them. 請問句中,用kill還是kills呢?

And, for germs and bacteria [that enter the body (when a person eats food)], there are [acids in the stomach (that kill them)].
that enter the body是形容詞子句，修飾前面的名詞germs and bacteria。提供其包括的範圍。
when a person eats food是副詞子句，修飾前面的形容詞子句。提供發生的時間點。
that kill them是形容詞子句，修飾前面的名詞片語acids in the stomach，提供範圍。
名詞片語acids in the stomach的主角是acids，in the stomach是修飾它的介系詞片語。
形容詞子句的動詞kill依據其修飾的名詞來決定「數」，acids是複數，故kill不能加s （第三人單數時，才可加）。
這句是複雜句(complex sentence)。

Q1: 請問用kill還是kills?
kill

Q2: that是關代，當主詞，先行詞是stomach還是acids呢？
acids

Q3: And, for germs and bacteria that enter the body when .... 句中enter前面為什麼需要放個that呢? 將that去掉可以嗎？
that是形容詞子句的主詞，不能省略。
for xxx是一個介系詞片語，移到主要子句的前面，表示它是副詞性質的修飾語。修飾述部。
There are acids in the stomach = Acids are in the stomach.
英文寫作在講存在時，常用there is/are表示。主詞是acids，there是主詞補語。所以動詞根據後面的主詞變化。

Q4: And, for germs and bacteria that .... 後看不懂句子為什麼可以這樣寫？
重要的資訊會用倒裝的方式，移到主要句構之前。for xxx, there are acids to kill them.=對於xxx，有酸可以殺它們。「句焦點」是胃酸可以消滅的對象。

我們也可改寫成：In the stomach, there are acids that kill germs and bacteria that enter the body.
這樣胃酸所在的位置就成了「句焦點」。

there are acids==being the subject clause and that being relative pronoun of there are acids; for being plural in number . therefore,the predicate should be kill. Yip

Q4: And, for germs and bacteria that .... 後看不懂句子為什麼可以這樣寫？
for = in relation to (至於)
這樣寫, 應該是接上文, 說及從另一途徑(例如呼吸道)進入身體的病菌 (註:沒有維生素).

There is only one independent (main) clause and not any dependent clauses in the sentence, so it is not a complex sentence.

Q1: 請問用kill還是kills?用kill
Q2: that是關代，當主詞，先行詞是stomach還是acids呢？主詞是 acids
Q3: And, for germs and bacteria that enter the body when .... 句中enter前面為什麼需要放個that呢? 將that去掉可以嗎？因為that是修飾germs and bacteria
Q4: And, for germs and bacteria that .... 後看不懂句子為什麼可以這樣寫？英語的句子都是如此地表達的,中譯:當一個人吃了食物因為有了細菌和維生素進入人體......