He is working for five hours. (Correct the following sentence)

The original sentence "He is working for five hours" is grammatically incorrect because the present continuous tense (is working) is used to describe an action happening at the moment of speaking or around the present time. However, when we mention a duration (like "for five hours") indicating an action that started in the past and continues up to the present, we must use a perfect continuous tense.

The correct tense to use in this context is the Present Perfect Continuous Tense. This tense is formed with "has/have been" + the present participle (verb + -ing). It emphasizes the duration of an action that began in the past and continues into the present.

Uses of 'For' and 'Since':

    
• For: Used to indicate a period or duration of time (e.g., for five hours, for three days, for a year). It answers the question "how long?".
    
• Since: Used to indicate a specific starting point in time (e.g., since Monday, since 2010, since 5 o'clock). It answers the question "when did it start?".

In the given sentence, "for five hours" specifies the duration, making the present perfect continuous tense the appropriate choice to convey that the action of working started five hours ago and is still ongoing.