"Ik opende het raam."
Translation:I opened the window.
So the present tense of this sentence (that I've seen on Duolingo) is "Ik doe het raam open"... and here the past tense is "Ik opende het raam"... how come the past tense isn't "Ik deed het raam open?"
Why does the "doen" disappear?
'Doen' doesn't disappear, a "different" verb is used. Both 'openen' and 'opendoen' mean 'to open', and are (most of the times) interchangeable.
('Ik deed het raam open.' is perfectly fine as well!)
'Opendoen' is a separable verb. It's split into two words ('doe' and 'open' in the present tense and 'deed' and 'open' in the past tense). As Merrie already said, it's a different verb from 'doen' altogether and they are mostly interchangeable.