While using present continuous tense should I end it with time point?

Question

While using present continuous tense should I end it with time point (based on the grammar rules)?

For example:

• He is studying.

or

• He is studying now

?

I was told in the past the when having present continuous tense I should add time point such as: now, right now, at the moment, or at present etc. But practically speaking I don't see that people follow such rule. What is the explanation for that? There are three possible choices: 1) I was told wrong thing 2) people wrong, 3) I didn't understand what I was told.

Answer 1

Here is what MyGrammarLab elementary by M.Foley and D.Hall says about the use of time expressions in question:

You see, we often use such words and phrases, not always - only when it's necessary.