As time passed by, I could ride faster than before. Bike riding became fun.

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In the second sentence, what is the part fo speech of 'fun'? Is 'fun' a noun or an adjective?

adjective

Max is right. It's the predicate adjective after the linking verb "became."

To determine the part of speech of a word like 'fun', you can use a few different strategies. One way is to examine how the word is used in the sentence and consider its role and meaning.

In the sentence "Bike riding became fun," 'fun' is being used after the verb 'became' to describe what bike riding became. In this context, 'fun' is acting as a predicate adjective, which means it is modifying the subject ('Bike riding') and describing a quality or condition of it.

Another way to determine the part of speech is to look it up in a dictionary. According to most dictionaries, 'fun' can function as both a noun and an adjective. As a noun, it refers to enjoyment or amusement, whereas as an adjective, it describes something that provides entertainment or enjoyment.

So in the sentence you provided, 'fun' is functioning as an adjective. It describes the quality or condition of bike riding, indicating that it is enjoyable.