Sometimes a preposition doesn't really have a meaning on its own; it's "demanded" by a verb. Another example of this is that "soñar con" means "to dream about." I studied Spanish formally for seven years, and every textbook and dictionary I used treated "verb + preposition" as its own vocabulary unit. It's a shame Duo doesn't provide this kind of idiomatic information in dictionary hints. One good web resource for this kind of stuff is wordreference.com
That's one of my only complaints about DuoLingo. However, no language learner should use solely one tool to learn any language. It's important to glean from every possible source. It just gets frustrating sometimes because when I'm on a good streak, sometimes I don't want to exit the app or open a new tab to spend another 10 minutes studying why the phrase has to be spoken a certain way. A lot of these phrases and word endings seem like they could be confusing to people unfamiliar with Spanish. Luckily, I took 2 years of it in high school and I have a good base.
And one more site you can check out as well:
I agree with undeadgoat but I think the english word "to" can be translated in spanish with the word "de" (but not always) for example: I tried to run " yo traté de correr", I tried to speak "yo traté de hablar",etc. I'm a native spanish speaker. But this is not a dequeismo because in this sentence we need the "de" if you don't put it, the sentence is incorrect.
I prefer, in general, to take the "to" from the English infinitive rather than the "de"
Remember, for example, ir = to go, ir does not equal go.
Ellos habían tratado de ir
tratar de = to try tratado de = tried
de is essentially untranslated, but it necessary to turn tratar into try. Without it, tratar means to treat.
Dealing with prepositions that are required with some verbs for particular meanings in Spanish as unstranslatable or simply part of the verb (or the verb with that meaning) will save you a lot of heartache versus figuring out how to translated it.
(Ellos) (habían) (tratado de) (ir)
They ....had ........tried...........to go
I put "A" instead of "DE" thinking "A" means "TO." In this case, is it best to just accept "Tratado De Comer" as a whole that means "Tried To Eat?" Every once in a while I'll put an "A" where I am supposed to put "DE." Is there some sort of rule that can help me avoid doing this?