It is skipped always, unless in some complex sentence it is necessary for understanding who is doing what (but even then it is not really obligatory): "Ja jem jabłko, a ona gruszkę" - I am eating an apple, and she [is eating] a pear. It is obligatory when the point of the phrase is to stress some kind of contrast between the persons involved.
It is a bit tricky... When you say "(Ja) Jem jabłko" (singular) - it means "I am eating an apple". From the point of view of Polish grammar an expression "I eat an apple" makes no sense, because you cannot eat one apple for several hours (well, you can, but polish grammar would treat it as present continuous), nor you can eat the same apple again and again. Only when you specify "Jem jabłko każdego dnia" it would translate to Simple Present "I eat an apple every day".
But when you say "Jem jabłka" (plural) it means either that you eat apples habbitually, hence it translates to "I eat apples", or that you have several apples to eat in a row: "I am eating apples". You have to make it out from the context what would be the best translation. Sometimes, there are added words for better clarification - like: "Ja zawsze/zwykle/czasami jem jabłka" - "I always/usually/sometimes eat apples" or "Jem teraz jabłka/jabłko" - "I am eating apples/an apple now".
There are also other verbs for eating: "jadać" - "to eat from time to time" , "jeść" or "zjadać" - "to eat now /to eat continuously" and "zjeść" - "to 'have eaten'" (yes, I know that the last one it is not correct in English, but this verb is used to create only perfective tenses: it has prefective aspect).
Maybe one of these would be helpful:
but I cannot guarantee any usability of these, I just used google search to find them, although the last one seems to be a pretty in-depth.
I forgot to put a 'n' for "I am eating an apple (Jem ja jablako) and It counted me as wrong. What?
- The phrase is in indicative mood. In indicative mood the preferred word order is Pronoun - Verb - Object, while the order Verb - Pronoun - Object, that you have used, may be used in interrogative mood. Besides, where possible, in Polish we try to avoid sequences that resemble stuttering, like "ja jabłko".
- In Polish "apple" is "jabłko". You wrote "jabłako", which is similar to "jabłoko" ("яблоко") in Russian.