A number of compsitions ??

(1)some candidates to use grammatical structures and vocabulary which were
beyond their ability to handle successfully.
一些考生力有不逮地, 未能成功地運用英文法結構和英文字彙。

(2)revealed a tendency among some candidates to use grammatical
structures and vocabulary which were beyond their ability to handle
successfully.
顯示出一個趨勢，就是一些考生力有不逮地, 未能成功地運用英文法結構和英文字彙。

(3)A number of compositions this year revealed a tendency among some candidates to use grammatical structures and vocabulary which were beyond their ability to handle
successfully.
從今年不少篇英文作文中，顯示出一個趨勢，就是一些考生力有不逮地, 未能成功地運用英文法結構和英文字彙。

(4)A number of compositions this year, while relevant, and certainly not
memorized, revealed a tendency among some candidates to use grammatical
structures and vocabulary which were beyond their ability to handle
successfully.
從今年不少篇雖然貼題和並非默寫的英文作文中，顯示出一個趨勢，就是一些考生力有不逮地, 未能成功地運用英文法結構和英文字彙。

A number of compositions this year, while relevant, and certainly not
memorized, revealed a tendency among some candidates to use grammatical
structures and vocabulary which were beyond their ability to handle
successfully.

如果要按句譯：
今年不少篇作文，雖然貼題和並非默寫的，顯示出一些考生使用他們成功地運用力能以外的文法結構和字彙的趨勢。

意譯：
今年不少篇作文，雖然貼題和並非默寫的，顯示出一個趨勢，就是一些考生使用他們不能成功地運用的文法結構和字彙。


While the original sentence is complex, it is perfectly grammatically correct. The clause "while relevant, and certainly not memorized" is an adjectival clause stating the author's opinion that the compositions were relevant to the topic and were not memorized and copied as is. However, the author suggested that the grammatical structures and vocabulary used in the compositions were probably too complex for the candidates to be able to handle successfully.

The base sentence is
A number of compositions this year, while relevant, and certainly not
memorized, revealed a tendency among some candidates to use grammatical
structures and vocabulary which were beyond their ability to handle
successfully.

The compositions are relevant and certainly not memorized.
The tendency is among some candidates to use X.
X is grammatical structures and vocabularies which were beyond their ability to handle successfully.

您好，我推薦您一個非常不錯的英語學習網站，里面有很多非常不錯的學習資源，您能從中得到不少幫助：

http://www.hkenglishstudy.info

祝您好運!

這是一句complex到我懷疑他自己都唔知自己想講乜，希望唔係你自己寫完試下人地解唔解得掂。係就唔該寫過，簡單啲，有很多grammar(verb tense)錯。

A number of compositions this year,
今年內在很多篇作文，
while relevant, and certainly not memorized,
雖然寫得切題，並且不能被記下
[我估應該係and certainly not memorable=並且不令人印象深刻]
revealed a tendency among some candidates to use grammatical structures and vocabulary which were beyond their ability to handle successfully.
反映有些學生們對文法結構和詞彙使用的成功駕馭上都出現力有不逮的趨勢