lieven:

If you think "because it happened in the past", then drop it.

good point. but the question now arises: usually the things which have
happened in the past, haven't been completed as well ?

You're not building sentences based on logical truths; you're building context. Something may have been completed in the past, but are you TALKING about it being completed?

For instance, wa'Hu' pItSa' vISop yesterday, I ate pizza. It says that pizza-eating happened yesterday, but in that sentence pizza-eating was NOT completed. It is a statement of general truth. General truths are not completed; the FACT of my eating pizza was not completed. Pizza-eating happened yesterday.

But then there's wa'Hu' pItSa' vISoppu' yesterday, I ate pizza. English makes no distinction, but it means something different in Klingon. Yesterday there was pizza-eating, and I'm telling you that I ate and finished eating it. This is not a general truth; this is describing a specific event. Yesterday, pizza-eating happened and completed.

The former version might be an answer to the question wa'Hu' nuq DaSop what did you eat yesterday? The latter to the question wa'Hu' nuq Data'pu' what did you do yesterday?

-- 
SuStel
http://trimboli.name