文法問題，which帶出的形容詞字句?

which修飾"this lack of repair work"是一組詞意的，留意不是單一的字，而是全句的組成包含一些"片語"或"子句"等；你應該注意的句子主句是"It was"...因此which後的was其實是等同第一was後的全段作出"附加"註解的作用。

文法(規律)只是結構，句法才是傳意。

And it was "this lack of repair work" "which" was directly responsible for the explosion.
"which" is a determiner, pronoun used to add more information about a thing (eg:-repair work, a noun) nearest to it .It also depends on the logical main word related, which is "work"; that may also be adv.cl of relation, conjunction.
Hence, "which" describes the noun "work" nearest to it.

修飾那一個需視其邏輯主詞，本句which所引導關係子句是修飾lack，因of repair work是介詞片語修飾lack，我們称其為後位修飾，例There is something wrong with your homework.
但若本句改成 it was a piece of bread which was rotten.即是修飾最鄰近的名詞bread所以一個句子要看邏輯主詞在那才決定

這是英語常見的cleft sentence, 用於强調句子, 通常是原句的主詞.
結構上:  It (dummy-it) + verb-to-be + [subject] + [relative clause]  
原句:  This lack of repair work was directly responsible for the explosion.

And it was this lack of repair work that was directly responsible for the explosion.

正如Jenkins指出的，"this lack of repair work"這群字是合在一起思考的，因為它所代表的事情正是後面那個子句的主詞，這是最簡單的常識都可理解的(因為，presumably，聽話的對象通常是人而非機器人)。
通常在口語裏，這討論就結束了，然而只要稍為嚴格一點，就發現which應改為that，因為which只能代表"名詞"，而that則可代表"子句"(一件事)。

This lack of repair work was directly responsible for the explosion.
使用突顯句型來寫時，就變成了：
It was "this lack of repair work" that was directly responsible for the explosion.
使用形容詞子句當修飾語，句焦點在前面。（通常用關係代名詞用限定性質的that)
What was directly responsible for the explosion was  "this lack of work."
使用名詞子句當主詞，句焦點在後面。

文法是我醫生的惡夢............