Ok so in the context of clothing, usar most appropriately means "to wear". Is that right? And vestir implies the act of putting on the clothes?
Think of it this way, Vestido (noun) is dress as in the article of clothing – and is masculine despite it is mostly women who wear them these days though that really was not the case years ago, just different types of dresses before (scroll down for full effect).
And Vestir is to get/be dressed (which is not quite the same as wear).
No.verbs are conjugated according to the person: 1st, 2nd, 3rd sing/pl, not according to the gender. Eles usam, elas usam.
why is boys USE pants wrong. Is it not also correct? Meninos usam calças velhos para limpar o chão
Yes, in such context it works, but i dont think that was the point Duo was supposed to teach. (Ps: calças velhAs)
The sentence does not have the article "O." I presume this is meant to be a general statement, as opposed to a specific set of boys. In English, "The boys wear pants" and "Boys wear pants" mean two very different things.
Slacks is not a commonly used term outside of the Southern hemisphere (occasionally and more formally in the US), but should also be accepted as correct, but sometimes all possible translations have not been included. You can send a message to the Duolingo team and I'm sure they'll correct this.