Why is ing added after these verbs 1.I'm visiting my friends. 2.He isn't writing a postcard. 3.Is she enjoying her visit? 4.Are they buying postcards? Are these sentences in the present continuous tense? If so, where can I tell? If not, what is the rule of adding ing after verbs? These sentences include negative sentences and interrogative sentences, and the verb is also added with ing. Should the verb keep its original form in negative sentences and interrogative sentences? Under what other circumstances do verbs need to change their ing forms?

Why is ing added after these verbs 1.I'm visiting my friends. 2.He isn't writing a postcard. 3.Is she enjoying her visit? 4.Are they buying postcards? Are these sentences in the present continuous tense? If so, where can I tell? If not, what is the rule of adding ing after verbs? These sentences include negative sentences and interrogative sentences, and the verb is also added with ing. Should the verb keep its original form in negative sentences and interrogative sentences? Under what other circumstances do verbs need to change their ing forms?

Gerund form should also be added (gerund is a noun verb)
eg：Making the bed is very simple!
"Making" here is not a verb, but a noun
In addition, "ing" should be added in the present continuous tense,
Generally speaking, the copula (am is are) can't appear at the same time as the verb,
Add "ing" after the verb
.I'm visiting my friends.
It's the future tense. Visit is an instant verb. It's the present tense
He isn't writing a postcard
He is not writing a postcard
.Is she enjoying her visit?
Generally speaking, the copula (am is are) can't appear at the same time as the verb,
Add "ing" after the verb
.Are they buying postcards?
Are they buying postcards?