The word <em>ice</em> is part of the subject <em>the ice</em>, which agrees with the verb <em>forms</em>. The subject is simple and requires the verb in the third person of singular, of the simple present tense (<em>to form - it forms</em>). The subject <em>comets</em> agrees with the verb <em>shed</em> (simple past tense). The word <em>tails</em> is part of the direct object <em>their glowing tails. </em>The word<em>that</em> is a determiner.
