英文文法一問。句字結構?

Leftovers from a restaurant meal may either be left behind to be discarded by the restaurant or taken away by the diner for later consumption.

主詞+兩個對等平行的述部(都是被動態動詞,用either ... or連結)
Leftovers may either be left behind or taken away. (簡單句)
= Leftovers may be left behind.
+ Leftovers may be taken away.

修飾語:
from a restaurant meal (介系詞片語,修飾前面的名詞leftovers,屬於形容詞性質的修飾語)
to be discarded by the restaurant (不定詞片語,修飾前面的述部may be left behind,表示目的: 這是下一階段要做的事.屬於副詞性質的修飾語)
by the diner (介系詞片語,帶出被動態動詞take在邏輯上的主詞,屬於副詞性質的修飾語)
for later consumption (介系詞片語,修飾前面的述部may be taken away,表示目的.屬於副詞性質的修飾語)

不定詞(infinitive)通常都會在前面加上[不定詞的記號]to
bare infinitive才沒有用to,屬於例外用法,如
感官動詞: I felt my heart "beat" violently.受詞補語用不定詞時,須用bare infinitive
使役動詞: What makes you "think" so?

The subjective phrase==Leftovers from a restaurant meal;
The verbal phrase as predicate==may either be left behind;
The prepositional phrases==to be discarded; by the restaurant; or taken away;by the diner; for later consumption. Yip