The Present
We need to ask 2 questions to determine if an action takes place in the present:
Action / state:
Did the action / state finish? The answer must be “No.”
Did the time period finish? The answer must be “No.”
Then, the questioned action takes place in the present. For the most part, in this situation we use one of present tenses.
Some time in the past I started my first cup of coffee as a part of my daily routine of drinking 2 cups of coffee every day, even though today I finished my action of drinking coffee, but I still continue to drink 2 cups of coffee every day periodically. So, this action has not yet finished. Also, the time period “every day” still continues every day. The time period did not finish. That is why we have used the Present Simple Tense here.
The Present Perfect Tense is a whole exception to this two-question rule. Please, refer to this interesting lesson: Present Perfect: Why “Present” when it denotes a past action?
He is working a lot today.
Did he stop working? No. Did the period of time finish? No. See today vs yesterday in Introduction to Tenses: Finished vs unfinished vs unspecified period of time. So, we use Present Continuous here.
Here is another example of a state:
His face is pale.
His face started being pale some time in the past and this state of his face (being pale) has not yet finished. So, we have used the verb be in the Present Simple. Regarding the time period, we can infer that it’s not yet finished from the context.
He is drinking a coffee now.
Did he finish drinking coffee? No.
Did the period of time finish (now)? No. See now vs then in lesson Introduction to Tenses: Finished vs unfinished vs unspecified period of time.
Well, we have answered no and no to the above questions. That is why we can use a present tense here, Present Continuous to be precise.
Be careful with the negative sentences
He is not drinking a coffee now.
In this case it’s just the same sentence, but in the negative form. And it is now tricky to ask these two questions. In this case let’s rephrase the questions to our needs:
Did he finish his “not drinking” a coffee? No.
or
Did he finish his inaction? No.
or
you can just flip a negative sentence into a positive one (I call them so), and then apply the original 2 questions. Actually, the polarity of the sentence does not affect the time concept. Mathematics is actually very close to linguistics. This is going to be my next research.
He is not drinking a coffee now. —> he is drinking a coffee now.
Did he finish drinking coffee? No.
Did the period of time finish (now)? No.
Interestingly, the present is always dynamic, it is constantly moving together with the current time, with every micro second of it. Past is “before now, before current time” or “before present,” and future is always “after now, after present, after current time.” Past and Future are static, they are statically fixed: at 5 tomorrow, from 5 to 6 yesterday etc.
More on Introduction to Tenses: