In general, all you need to do to insert a comment is to click (insert the cursor) at
any point in the text, and then click the appropriate button on T.A. Toolbar.
Some buttons, however, address writing errors that students might find confusing
if the error itself were not specifically marked in some way. T.A. Toolbar has
therefore been designed to highlight in yellow certain kinds of errors. The
special buttons that do this automatic highlighting are:
the first Pronoun Agreement button;
the first Subject-Verb Agreement button;
all of the first Shift (Person, Tense, Number) buttons; 
and
all of the yellow buttons.
When using these buttons, simply double-click the offending word, then click the
appropriate button on T.A. Toolbar.
The Plagiary Detection button 
is also a special case and is discussed here.