Is "he is replacing the window soon" really a correct sentence? Shouldn't it be "he'll replace the window soon"(DL accepted this)
They shouldn't have accepted that answer because in order to construct the future tense you must use the verb "werden". "Er wird bald das Fenster ersetzen" This sentence is in present tense and can therefore be translated as either, "He replaces the window soon", or, "He is replacing the window soon" which sounds more natural to me.
German mostly uses present time to express the future - the construct with "werden" is actually pretty rare. However, I'm little surprised (in a positive way!), that DL accepted the translation with "will".
To sum up: Both warforgad's sentences are correct. In English the sentence "He is replacing the window soon" can be used, when it is a planned action and it's sure that he (the worker) will come and replace it.
Adverbs of time go in English either at the beginning or the end of a sentence.
So German doesn't distinguish between transitive and intransitive words in any way?