The students in Hong Kong are likely to commit suicide, which is a kind of social issue that the community have to face. 關於這句句子的結構?

1. (likely 用得糟糕) yes, but we can use another word "prone" to replace "likely".
2. (原句 寫得不通) yes, the sentence can be rewritten to be more concise, but let's retain the original structure as much as possible for sake of exploration.

Part-A:. The students in Hong Kong are prone to commit suicide. (SVC)
S- the students in Hong Kong, where "in Hong Kong" is an adjunct.
V- are (copular or linking verb)
C- prone to commit suicide (subject complement), where "to commit suicide" is a complement (not object) of the predicative adjective "prone".

Part -B: Apposition
That (A) is a kind of social issue.
"That the students in Hong Kong are prone to commit suicide" is a noun clause, subject of above sentence.
~ cleft sentence with dummy "it" ~
It is a kind of social issue that the students in Hong Kong are prone to commit suicide.
(in this case, that-clause is an appositive clause, renaming the noun phrase "a kind of social issue")

Part-C: Relative clause
It is a kind of social issue that the community has to face.
("that the community has to face" is a relative clause modifying the noun phrase "a kind of social issue")

1. 原句 寫得不通.最好不要研究它.
(a) likely 用得糟糕
(b) a kind of social issue 渙散無力
(c) which is 讓句子看得冗長

2. 應該寫成:

High school student suicide is an urgent social issue that the HK community has to face.

主要子句＋修飾語1＋修飾語2
主要子句：The students in Hong Kong are likely to commit suicide
修飾語1: which is a kind of social issue （修飾前面的主要子句，關係代名詞which以前面的主要子句全句為修飾對象。文法上稱這種關係子句為sentential relative clause, which = and that；有別於以名詞單字或片語為修飾對象的adnominal relative sentence）
修飾語2: that the community have to face（修飾前面的名詞issue。是限定性質的修飾語。關係代名詞that是形容詞子句動詞face的受詞。）

主要子句的動詞是一個片語：are likely to commit。are在形式上是主要動詞，但實質上帶意義的動詞是commit，suicide是其受詞。即此動詞片語是：be + 不定詞的結構（中間夾著一個副詞likely當修飾語）。表面上是「現在式」，但是「未來式的相等語」。指的是即將要發生的行為。

更正:石破天驚在意見欄說得有道理,我上面的解釋錯誤.
... are likely to commit suicide裡的are是主要動詞,likely是當作主詞補語的形容詞.不定詞to commit suicide是修飾likely的修飾語,屬副詞性質.

likely + to do＝有可能(做...)的
<新世紀英漢辭典>likely p.812 當形容詞用法解釋2
He is likely to come.