<p>In the 1960s, Cesar Chavez (organized) migrant farmworkers, (many of them) Mexican Americans, (into) a labor union that supported nationwide boycotts (of both) lettuce and grapes.</p>
<p>Why is many of them correct? Doesn't it need a verb?</p>
<p>Perhaps as (a consequence of) warfare (that erupted) when natural resources became scarce, many of Easter Island's large stone statures, called Moai, (have been toppled) by the islanders (themselves) three centuries ago.
Shouldn't consequence be plural? What is wrong with "have been toppled?"</p>
<p>The illuminated manuscripts in the rare-books collection, (all more than) five hundred years old, (are) the (products of) a scribal art (long since lost).</p>
<p>“long since lost” is the correct form. To check the correctness of the phrase “long since,” use this technique: replace “long since” with “long ago” and move it after the past participle. For example:</p>