Arduous is an adjective.

Here are some examples:

"It was a long and arduous journey."

"It was not too arduous a task for a Sunday morning."

"The mountain climbers tackled the arduous terrain."

The word arduous is an adjective. Her journey was long and arduous, but it ultimately strengthened her character and made her a better person.

Writing this sentence was rather arduous

'The football practices were arduous, but the players knew tough practices were necessary in order to build a successful team.' The word arduous is an adjective.