The correct answer that you give here is, in fact, grammatically incorrect in English. The grammatically correct answer in English makes use of the present perfect tense to incorporate past and present, with the expression 'for six months' . The correct answer should be: 'He has been training alone for six months'. Please correct DUOLINGO
It's not required; the sentence is not ungrammatical without it. It might be better phrased another way, but there are a lot of sentences that fall into that case and they all have their possible uses. For example, you might use this particular construction in the following way:
"Master chess player So-and-So follows an unusual training regimen, but has nonetheless risen to the top of the charts. For six months he trains alone, playing only against himself and his thoughts. During the rest of the year, he teaches the game to local seagulls. "Their wings really get in the way," So-and-So admits, adding that they "rarely seem to grasp the fundamentals unless the pieces are made of fish". Still, he credits his phenomenal success in professional chess with these unusual efforts, claiming that his own playing skills are improved through having to teach impossible students."
Sure, this is a specially constructed case, but that's fine - the question is grammar, not commonality. There are also others that feel a lot less forced. "For three months, I take classes, and the rest of the year I do fieldwork" is one example. Or, "For two hours I check my emails, and then for thirty minutes I schedule the week's meetings, and then for four hours I write code..." also works. They get across the idea of a repeating event just fine, and none of them is improperly grammared.
Actually, your examples merely prove my point. The sentence is fine within an explicit context. When the context is lacking, the reader is confused: Does the sentence mean that the person has been training alone for the last six months? Or does the person train alone for six months a year? To ensure that the reader understands the context, it is very easy to add the context to the sentence so that the user will understand which verb tense to use. Leaving the context implied and unclear means that you expect the reader to understand which context you are referring to.
Yeah, but can you really say that this is the only sentence which is unclear out of context? Duo has a LOT of sentences (in every language) that are silly or can have multiple meanings or uses - that's not really super important, as long as you can understand what the words mean and how they fit them together.
If this were teaching prose or style it'd be a terrible example. As a basic structure it's... unusual, but fine.
For the longest time i saw the word "trains" and could only think of the transportation! I was completely baffled. Me two minutes ago: What horrible sentence is "trains alone!?" Realization sets in "oh." Time to put Duolingo down. Haha i think my brain is feeling a little fried.
Allena is present tense, not continuous present tense (gerund) "allenando", and duolingo is particular that you maintain the tense of the verb. Also "For six months, he is training alone" could work but sounds funny, I would prefer "He is training alone for six months". My 2 cents.