ているところだ

ているところだ says you are in the middle of an action at this moment. It goes after the て-form of a verb, and the usual reason to say it is that someone has asked what you are doing or interrupted you.

ているところ or ている

れぽうとレポートをかいている書いている can mean "I am writing a report" or "I write reports" as a job or habit. ところ narrows it to the one action going on right now.

It only works with an action that is under way. A ている that describes a resulting state – まど窓がひらいている開いている, けっこん結婚している – does not take ところ.

• × 窓が開いているところだ → 窓が開いている

Before, during, after

The form of the verb before ところだ sets the moment. The dictionary form is just before the action, ている is in the middle of it, and the た-form is just after.

In the past

In a past sentence ところだ becomes ところだった, which says what you were in the middle of when something else happened.