What are the key differences between present simple and present continuous tense?

The present simple and present continuous tenses are both used to talk about actions or states happening in the present, but they have key differences in terms of their usage and structure.

1. Usage:
- Present Simple: The present simple tense is used to describe habitual actions, general truths, and permanent situations. It is also used to talk about scheduled events or timetables.
Example: "I go to the gym every day." (habitual action)
Example: "Water boils at 100 degrees Celsius." (general truth)
Example: "She works as a teacher." (permanent situation)
Example: "The train leaves at 9 am." (scheduled event)

- Present Continuous: The present continuous tense is used to describe actions happening at the moment of speaking, temporary situations, and future arrangements.
Example: "I am currently studying for my exam." (action happening at the moment)
Example: "He is staying with us for a few days." (temporary situation)
Example: "We are going to the movies tonight." (future arrangement)

2. Structure:
- Present Simple: The present simple tense is formed by using the base form of the verb (infinitive) for all subjects, except for the third person singular (he, she, it), where an -s or -es is added to the verb.
Example: "I work, you work, we work, they work." (base form)
Example: "He works, she works, it works." (verb + -s/-es)

- Present Continuous: The present continuous tense is formed by using the present tense of the verb "to be" (am, is, are) followed by the present participle (-ing form) of the main verb.
Example: "I am studying, you are studying, we are studying, they are studying." (verb "to be" + present participle)
Example: "He is studying, she is studying, it is studying." (verb "to be" + present participle)

3. Duration:
- Present Simple: The present simple tense does not indicate the duration of an action. It focuses on the action itself without specifying when it started or ended.
Example: "I play the piano." (no indication of duration)

- Present Continuous: The present continuous tense indicates that an action is happening at the moment of speaking or around the present time. It implies a temporary duration.
Example: "I am playing the piano." (action happening at the moment)

In summary, the key differences between the present simple and present continuous tenses lie in their usage, structure, and indication of duration. The present simple is used for habitual actions, general truths, and permanent situations, while the present continuous is used for actions happening at the moment, temporary situations, and future arrangements. The present simple is formed with the base form of the verb, while the present continuous is formed with the present tense of "to be" and the present participle.