Answer:
An infinitive phrase as the subject
Explanation:
Infinitives are a type of verbals (verbs forms that do not act as verbs) that function as adjectives, adverbs or nouns in a sentence.
It is formed with the word "to" + the simple present form of a verb; for example, to + play. Infinitive phrases, then, are phrases containing an infinitive that is followed by any objects, and/or modifiers of that infinitive.
The sentence "To win an argument is seldom a desirable goal" contains an infinitive phrase, which it's "To win an argument", and such phrase is acting as a noun, more specifically, as the subject of the sentence because it is the main thing being described.
