Translation:My yard has a tree and a lot of grass.
I put much grass, instead of lots of grass, and it told me that was wrong. I'm assuming that it is a correct option, but just hasn't been marked as such in the system?
Yes there are many options supposed to be correct that they just don't accept. Little by little, as people report these kind of mistakes, their store will increase and losing hearts through right answers will be more difficut to happen. So, when that happens again, its a good idea to report that. Then, you help others and help portuguese section on duolingo be out of beta sooner.
So "jardim" can mean both grass and yard? Is there any point where a yard is no longer classified as a "jardim"?
We don't usually use 'much' in affirmative sentences in English, so 'much grass', while not exactly wrong, isn't something people would say.
So, grama is grass in general / all sorts of grass? Because in english, grama grasses are a genus of (mostly tall) grasses. (I guess they're mostly from the Americas, so it makes sense.)