Why "To make this work, the engine should be placed in the car after the chassis." is a correct sentence?
Hmm. I haven't studied this recently, but I think of it sort of as a kind of conditional. "In order to make something happen, these steps should be taken." "In order to make this work, the engine should be placed in the car after the chassis." It's implied that placing it in before t
You are right to try to avoid passive constructions. This is a good example of why we all should. The introductory infinitive clause has a sort of phantom subject, what you have rendered as "we" in sentence 2. Sentence one seems to mean that the engine has to do something, which is obviously absurd. I am not saying that the sentence is strictly ungrammatical, or that the erroneous meaning is expli