I keep getting confused as to when an article should precede a noun. I think I can understand the difference between "el cangrejo bebe la leche" and "el cangrejo bebe leche" but let's say the crabs where plural. I would think there is a difference between "Los cangrejos beben leche" and "Cangrejos beben leche" in that the former refers to a certain group of crabs while the latter refers to crabs in general. But earlier there was a question to translate "Los caballos son animales" which translated to "Horses are animals" (horses in general, it would seem).
I think they're using definite articles so you get used to them and to help you remember gender.
Using the definite article is still pretty general (e.g. "The female of the species is more deadly than the male."), although rather stuff-sounding in English, unless you give more information (e.g. "The movie that we saw yesterday," ).
If you want to be specific about the noun, you'd use the words for this/these (este/esta/estos/estas) and that/those (ese/esa/esos/esas).
Why do people keep getting upset about the sentences being "absurd"? We are here to learn Spanish, not biology. So what if the system gets the dietary preferences of crabs incorrect? I am sure you can find all the information you could ever ask for on what the typical dietary intake of various animals are by going to Wikipedia. This is Duolingo.
Dude, look through the string of comments above here. Each one of them deals with the absurdity of crabs that drink milk. Yet, when a reasonable question is asked on grammar and when to use articles before nouns, it goes unnoticed because everyone is too busy expounding on how absurd the sentence is.
What exactly is the purpose of these discussions? I must have been mistaken but I was labouring under the impression that it was to get clarity on language issues.
You're being too serious here. If you're so keen in learning the Spanish, I suggest you quit this app, delete it, go for Spanish classes. I understand that you're here to learn however wouldn't the world be a better place if we just inject some joy or laughter while learning? As for most of us, it further enhances our learning. If you are so against all the jokes being made, please, I rather you take up outside classes. Cheers.
Having "learned" Spanish grammatically in University I can tell you that it is not the way to learn a language. The best way is speaking, next is listening, then reading. I have less trouble speaking to people in Spanish than I do with simple grammar on this site. They are completely different types of memory.
This phrase being grammatically correct isn't enough to make it a useful tool. I read "the crab drinks milk" and wonder if maybe that has some meaning in some Latin country like when we say "when pigs fly".
Imagine learning English and coming across this sentence: "The commenter on Duolingo with a stick up his arse will learn Spanish when pigs fly". It would be bewildering.