See below the explanation of my annotations:

1) Text Text   (this is the "insert text" annotation in Adobe Acrobat)
       ^        
-- no commentary = revise either punctuation (usually commas) or articles; sometimes difficult to see in the text, so make sure to open the comment bar

2) Highlighted text
-- no commentary = a fairly straightforward mistake in grammar or vocabulary; can also denote repetition

3) Text between. Two sentences
-- "Missing/weak/improve link" = the two sentences seem only loosely connected. Fix this by revising the sentences, using proper linking expressions, joining the sentences together, substantially revising, or changing the organization of the paragraph.

Additional Exercises Answers

Suggested answers for additional exercises. Multiple correct answers may be possible for some of the exercises.