If you have read the post before this one, you are aware that -고 있어요 (-go isseyo) is used to express an action in progress something similar to ‘verbing‘ like eating, dancing etc.
This verb ending can also be tensed (past progressive). So if 쓰고 있어요 (Sseugo isseoyo) means ‘is writing’ then 쓰고 있었어요 (Sseugo isseosseoyo) means ‘was writing’. Please note that the tense is applied not on the base of the verb but to the ending 있.
Negative expression can also be made in either on the verb used or on the entire expression itself. Using the example above, if you want to say is not writing then you can say 쓰고 있지 않아요 (Sseugo ittji anayo). Sounds familiar right? This has been discussed in my previous post on negating verbs. This is the long negative form. Now if you want to say something like ‘manage to get by without writing’ or ‘not writing at this time’ then you can say 쓰지 않고 있어요 (Sseuji ango isseoyo).