Is it helpful to learn a language from English ,then to learn English from that language?
Example: I am learning Spanish and just started learning English from Spanish. Is this a helpful thing to do?
It's called doing a reverse tree. After I finished my Turkish tree, I did English from Turkish. It had a lot more vocabulary in it, and because there is not a one to one mapping between Turkish and English, some of the translations were really challenging. They expected the use of proper Turkish in the answers. That was fun. It was also kind of irritating because there were a lot of English grammar questions that I didn't need. On the whole, I enjoyed it, and I recommend that you try it.
Yes! I haven't done it yet (as there's no English for Swedish course), but I'm sure it is.
The way Duo is set up you'll get more writing exercises in your base language. Meaning learning Spanish from English you'll learn to read Spanish and type the English translation. In the "reverse tree" English from Spanish you'll have more opportunities to type in Spanish. (Plus some added vocabulary here and there).
It's not harmful but I think your time would be better used by using your Spanish in real life, passively or actively, online or offline.
Doing the Spanish reverse tree is especially helpful since the last update, when Duolingo added approximately 24 new skills. Those new skills include many fairly complex sentences, with a good mix of tenses.
Even in the earlier stages of the tree, you will encounter vocabulary that is not in the English to Spanish tree. And, as has already been stated, you need to produce a much higher proportion of Spanish sentences in the reverse tree.
Yes, it is very useful if you do it after completing the "original course". I studied English from Spanish (my native language) and it was nice, but there were not enough exercises about writing in English. Hence, I "learned" Spanish and it was excellent because the Spanish course has the same problem, too much translating from Spanish into English.