It's because of the focus in the sentence. It's not very clear even for us native speakers but in "Hoy lo voy a dejar ganar" the focus of the sentence is on " lo voy a dejar ganar", whereas in "Lo voy a dejar ganar hoy" the focus is on "hoy". It also depends on the emphasis you put in the words when saying it though.
I hope this help you understand, 'cause frankly not even Spanish-speaking linguist students get it on our first years.
The inconsistency is problematic. It makes it more difficult to understand these nuances of the language, since there's no pattern. More important, if English does not shift focus with word order the same way that Spanish does in this particular case, then it makes no sense whatsoever for Duo to insist that only one English construction is correct.
Also, for clarity, can someone explain what "focus" means in this context? Is it about emphasis? For example, one might have been asked, "When are you going to let him win?" versus "Who are you going to let win this time?" Is that what we're discussing here? If so, then it definitely makes no difference where you place the "today" in English. The emphasis will be obvious from the context and, if spoken, the intonation. Absent context in a written sentence, English still wouldn't care about placement. Otherwise, I'm not sure why we are setting the focus on one part of the sentence versus another.
The issue with 'le' vs 'la'/'lo' sometimes is not straightforward. Some uses are clear and have no alternative. However, this is not one of those cases.
The case is just in the border of both choices, and it seems that related verbs have their personal preferences. I would normally say 'le' for man and 'la' for woman, which is incorrect because 'le' and 'la' are different pronouns (indirect [CI] and direct [CD] resp.) This tells me that this case is complex even for me and I must consult official references.
I am copying the original text and providing a translation of the conclusion:
"Los verbos hacer y dejar, cuando tienen sentido causativo, esto es, cuando significan, respectivamente, ‘obligar’ y ‘permitir’, siguen la misma estructura que los verbos de influencia: «verbo causativo + complemento de persona + verbo subordinado». Tanto hacer como dejar tienden a construirse con complemento directo si el verbo subordinado es intransitivo: «Él la hizo bajar a su estudio y le mostró el cuadro» (Aguilera Caricia [Méx. 1983]); «Lo dejé hablar» (Azuela Tamaño [Méx. 1973]); y tienden a construirse con complemento indirecto cuando el segundo verbo es transitivo: «Alguien lo ayudó a incorporarse, lo estimuló y hasta le hizo tomar café» (JmnzEmán Tramas [Ven. 1991]); «El alcaide de la cárcel le dejaba tocar el banjo todas las mañanas» (Cela Cristo [Esp. 1988])"
Dejar, with the meaning of allow/let are built with CD when the following verb is intransitive (does not require a CD to have sense), and take CI when it is transitive.
Since ganar is intransitive (in this case), the correct use appears to be using 'lo'/'la. However, in practice, the choice of preference varies depending on the region (including small regions of Spain or certain American countries). Note however, that including the CD of the last verb changes the case (makes ganar transitive): hoy le voy a dejar ganar el torneo.
http://www.uam.es/personal_pdi/filoyletras/ifo/publicaciones/3_cl.pdf [in Spanish], page 11, last paragraph and followings.
Diccionario panhispánico de dudas (sort of official Spanish FAQ): http://lema.rae.es/dpd/?key=dejar
(Adding to Santi_Minstrel answer)
Yes. Some verbs take a preposition after them in Spanish, and some don't. There really aren't any good rules for when they are needed, so you just have to memorize them.
Here is a good resource.
We actually do something similar in English: show up, make up (a story), sit down, lie down, break up, .....
Lo siento. They used "le" instead of "lo". Try this one: http://spanish.about.com/od/sentencestructure/a/lo.htm
You can also use your Duolingo Vocabulary to search for info on any words that you have learn.
While using the Vocabulary, mouse over the searched word in the "Forms column" for info. Click on the same word, but this time in the "Word" column and it will give you examples and usage for the word or term in question.
"Thank you for your reply."
In your example - I'm just reading it again - apparently 4 years later - "He answers the question"
He = subject
answers = verb
question = direct object (what got answered)
Another example: I gave it to him
I = subject
gave = verb
it = direct object (it got given)
him = indirect object (he got it given to him)
Dejarlo. Actually I think this is a case of clitic climbing that should not be allowed. I would say hoy voy a dejarlo ganar. You're letting him win. You're not letting someone win "it". It's quite ambiguous. Although I should qualify that by saying that I am extremely tired and didn't think too much about it :/
Read Wazzie's comment below. I thought the same thing, but Wazzie reminded me that stop is "dejar de," not simply "dejar." For that reason, "dejar" should translate to "allow/let."
Lo = direct object (masculine) Le = indirect object La = direct object (feminine)
So, if you took out "lo," it would translate to something like: Today I'm going to let ? win.
Similarly, you could say, "Ellos no nos dejaron entrar." This would translate to: They didn't let us enter.
Lo, in the first sentence, and nos in the second serve the same function - telling us who received the action.
So this is another one of these sentences where we have to play the "Great Guessing Game"!
Literal translation: Today her (him?, it?) I am going to stop, (allow?, leave?) to win.
My guess: Today I am going to allow her to win.
Wrong! I have no idea how to tackle these!!