We use continuous tenses rather simple tenses:
when we talk about action, that is not finished:
She was writing a letter. (She was is the process of writing.)
She wrote a letter. (It is written.)
I have been writing a letter. ( I am still writing.)
I have written a letter. (I have finished writing.)
if we want to suggest that an action is temporary rather than permanent:
Why are you crying? (Crying is a temporary action)
Sensitive people cry quite often. (That is a common characteristic.)
My friend was living here. (For some time only.)
My friend lived here. (Permanently.)
we refer to a prolonged or repeated action rather that a single event:
My friend was looking at her watch.
My friend looked at her watch. (Once.)
They dived into the river. (Once.)
They have been diving into the river. (Many times.)
There are some verbs that are not used in continuous sentences: