This example illustrates how to diagram an infinitive phrase used as a noun in the direct object position of the sentence.  After the action verb, "began," draw the direct object line, as the whole phrase: "to open the box" is the direct object of "began" (began what?...to open the box).

Note it is diagrammed on a standard and the WORD (NOT PREPOSITION) "to" is placed on the slanted line, "open" on the horizontal line (open what?...box.  Box is the direct object of the infinitive, "to open).  Box is written after the direct object line and the article, "the," modifies box.  The infinitive phrase as an object is sometimes called "infinitive object construction" in some books.

Pamela quietly began to open the box.


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