Locate the complete gerund or infinitive phrase and identify its noun use.
"Her motive was to keep peace in the family."
http://www.chompchomp.com/terms/gerundphrase.htm
http://www.chompchomp.com/terms/infinitivephrase.htm
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The gerund or infinitive phrase in the sentence is "to keep peace in the family."
In this case, the phrase functions as the direct object of the verb "was."
The complete infinitive phrase in this sentence is "to keep peace in the family."
To identify its noun use, we need to look at how it functions in the sentence. In this case, the infinitive phrase is used as the subject complement of the verb "was." It renames or describes the subject "motive" and answers the question "what was her motive?" This means that the noun use of the infinitive phrase is as a subject complement.