Sentence Correction

Hello,

Could you please help me understanding the following concepts ?

Thanks.


1) OG 11 # 12

"A phrase must be introduced by a preposition, not a participle"

After gradually declining is RIGHT
Following gradually declining is WRONG


2) OG11 # 35

"A clause must be introduced by the conjunction that"

Investigation have found local witness to be difficult to relocate WRONG
Investigation have that found local witness to be difficult to relocate RIGHT

3) OG 11 # 36

"The infinitive must be preceded by a conjunction"

Any example ?

4) Modifying phrase, which .....

When does which refer to a noun that is not close to it ?