Throughout the course, I have failed tasks because I omitted "есть", countless times. I have some exposure to Russian from way back, so some of the things in the course seemed very easy when I first started - however ommitting есть kept leading to failed sentences.
This is not a perfect example - maybe this one sounds better with есть than without. But has anyone else had troubles with this, and what is the opinion of the native speakers in the group?
У женщины уже есть яблоко.
There are a very high number of sentences like this and I'm not sure if my old skills in Russian are simply wrong, or if the course uses ЕСТЬ when it's not really needed, in order to be overly clear for learners.
I am not a linguist and maybe there is a lot of exceptions but in my native speaker opinion есть can be omitted in sentences like "He is ..., I am..., they are..." etc. But in sentences like "I have... He has..." omitting есть is a mistake. In your example "У женщины уже есть яблоко" есть is necessary. (Actually, the sentence "У женщины — яблоко" exists, it called "неполное предложение" and we use it in context like "У мужчины есть груша, а у женщины — яблоко". It looks like more complicate grammar and I'm not sure that it proposed in duolingo).
есть is omitted in construciton as a default tense. when it's obvious.
я человек. ты женщина. (шутка).
saying я есть человек, ты есть женщина - sounds very dumb, because the meaning of есть here is a mere tense thing which goes by default as "есть". it's the "am/is" thing in english.
when there are possible multiple readings OR you wish to emphasize on the fact of HAVING, you do not omit it. i.e. it's a "have" thing in english.
for example: "у меня корова." =sounds bad when it has no context and it's not an answer to something. because it's not clear what is it with the cow? what is it about your cow? maybe you meant "у меня корова летает (doing sth)?". oh, so you meant to say you HAVE one, okay "у меня есть корова".
but within the context: "- у кого есть корова? - ну, у меня." here it's already obvious and not needed but perfectly well can be used still.
so, think what kind of "есть" is there. if it's "AM/IS", most likely it's not needed. if it's HAVE/HAS then most likely it is required.
please note, i made this explanation up just now while thinking about your problem, so i may have missed something. but in general i think i got your comfusion. you may propose various usage examples so we will see if they fit my explanation or not.
Though there is the verb "to have" (иметь) in Russian, it is rarely used to designate possession. Instead, Russian uses the preposition y (at) followed by the genitive, plus the word есть (is/are), plus the possessed object at the nominative. Examples:
У меня есть ключ. I have a key. Literally: At me there is a key.
У сестры есть кошка. The sister has a cat. Literally: At the sister's there is a cat.
У тебя есть компьютер? Do you have a computer? Literally: Is there a computer at you?
В этом саду красивые цветы. - The flowers are beautiful in this garden.
В этом саду есть красивые цветы.- There are beautiful flowers in this garden.
Книга на столе. - The book is on the table.
На столе есть книга.- There is a book on the table.