It isn't raining now.
It doesn't rain very much in summer.
Why in the first sentence do we use is but in the second we use does?
Can someone explain in general, please?
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The present continuous of any verb is composed of two parts - the present tense of the verb to be + the present participle of the main verb. We use the present continuous to describe an event or action that is happening right now: it is raining, the dog is barking, the sun is shining. We negate the present continuous by negating the verb to be - it is not raining, the dog is not barking, the sun is not shining.
The simple present uses the base form of the verb followed by -s: it rains, the dog barks, the sun shines. We use the simple present to express habits, general truths, repeated actions or unchanging situations, emotions and wishes: it rains very much in the autumn, the dog barks when it sees a stranger, the sun shines very much in the summer, we eat fish on Fridays. We negate the simple present with the appropriate part of the present of the verb 'to do' followed by 'not': it does not rain very much in the summer, the dog does not bark when it sees its master, the sun does not shine very much in the winter, we do not eat meat on Fridays. "Does not" can be shortened to "doesn't" in informal speech and writing.
The present continuous has the form of: "Subject + verb to be + gerund..."
So "It is snowing" or "I am eating." The negation of this is done quite simply by adding a "not", so "It is not snowing" or "I am not eating."
Now compare this to present simple, which refers to an action which happens regularly. Present simple has the form: "Subject + verb".
"I watch the Dodgers on tv" or "I go to the gym."
Contrary to present continuous, the negation requires a helper verb "to do" of the form: "Subject + verb to do + not + verb"
"I don't (do not) watch the Dodgers on tv" or "I do not go to the gym."
Different forms require different structures. I know it can be confusing, but once you've established the tense, it should be fairly straightforward to recognize how to negate that sentence when necessary.
First sentence is at present continuous because it is describing an action happening now. It is raining now, at the moment of speaking.
Second one describes a general action, that takes place in general, not only at the moment of speaking. It doesn't rain in summer, every summer, not just the one this year. Thus, the tense present simple.