"He did not want to eat."
Translation:Él no quiso comer.
well, it depends on the verb. tratar de + infinitive is just the way to say to try to do that activity. Yo trato de explicar esto. dejar de + infinitive = to quit doing something (usually a habit) Voy a dejar de fumar. Deber (alone) + infinitive = to should do that activity. Ud. debe deletrear con cuidado. Whereas, Deber de + infinitive = to probably be doing that activity. Marco debe de llegar pronto. Marco will probably be arriving soon.
And then there are other verbs that just go with "a" to be the complete thought. Aprendo a hablar espanol. Te ayudo a aprender.
For info on modal verbs, try http://www.spanishdict.com/answers/188323/some-spanish-modal-verbs
the overall issue with duolingo...its about progression but somehow there is a disconnect...one exercise and one can get everything right...perfect...the next it is like you knew nothing in terms of structure, grammar etc even if you are bilingual. I get the program is trying to teach analytical thinking but something is off.