"I am going to count them."
Translation:Las voy a contar.
Not in this context. You'd use "Les voy a contar" when you are trying to say that you will tell something to someone. For example:
- Les voy a contar una historia (a ellos/ellas/ustedes). [I am going to tell them/you a story.]
- Le voy a contar un secreto (a él/ella/usted). [I am going to tell him/her/you a secret.]
And so on. You could even say those same sentences without the subject if you know what you are talking about like this:
- Se la voy a contar. [se = ellos/ellas/ustedes; la = una historia]
- Se lo voy a contar. [se = él/ella/usted; lo = un secreto]
Or, if you want to be even briefer:
- Voy a contársela.
- Voy a contárselo.
Voy a contar a los hombres is ambiguous and could mean either "tell them" or "count them".
Voy a contar los hombres is not correct Spanish. You must have the «personal a»: http://spanish.about.com/cs/grammar/a/personal_a.htm.
Because Les= an indirect pronoun and there is no indirect object in your model sentence. 'los hombres'= the men=Direct object. Las/Los= direct object pronoun.
I can only tell you that it is a Spanish grammar rule, but take a look at this site where this concept is explained better.
There are two ways of placing the direct object. I think Duo seems to reject the one with the direct pronoun attached to the infinitive and I have no understanding why it is marked wrong so many times..
But read here, scroll down half way and there is information about the placement of the object.
Hope it helps.
We can't use 'ellos' because ellos is a subject pronoun and means 'they'. 'Los' or 'las' are object pronoun and mean 'them'. In the Duo sentence 'las' is used meaning all females. It is possible to translate 'ellos'/'ellas' as them but only as an object of a preposition, such as 'por ellos'/'por ellas'
If you need to review the Spanish pronouns try here: http://users.ipfw.edu/jehle/courses/pronoun1.htm