Locate the gerund or gerund phrase and identify its noun function in the sentence.

Thinking is hard work.

Gerund or gerund phrase:

Noun function:

Gerund or gerund phrase: thinking

Noun function: subject

Explanation:

Gerund is a type of verbal which means it is a verb with a different function. In case of a gerund, it functions as a noun.

Basically recognizing gerund in a sentence is easy, it always ends in - ing.

Though it should not be confused with present participle which also ends in - ing. Present participle acts as an adjective and it is a part of verb forms (e. g. present simple continuous).

In this sentence thinking answers to the question what (what is hard work), so that's why it is a subject.