Greek is a "pro-drop" language, meaning the subject is not necessarily expressed overtly. This is because it can be easily inferred from the verb endings (e.g. -ω for active,-μαι for passive). English on the other hand is not a "pro-drop" language, so the subject must always be present.
In Greek or Irish? ;) In Greek, you include the pronoun to put emphasis on the subject of the verb, even if it is subtle, otherwise there's no good reason for it.
Using the pronoun all the time when talking/writing is a quick giveaway of a non native speaker. :) That does not mean that we don't use personal pronouns daily all the time, but their presence is justified by the slightly altered meaning. For example:
Θα διαβάσω το βιβλίο μου = I will read my book.
Εγώ θα διαβάσω το βιβλίο μου = Ι'm going to read my book and I don't care what you do / if the world burns / leave me alone! :D
It is strictly a choice. Táim or tá mé 'I am'; táimid or tá muid (= 'we are') although one version will normally be preferred in any given dialect.
There is no difference in meaning - it is just a choice according to dialect and personal preference.
If you want to emphasise the person they have extra endings that can be added, but they can be added to either the synthetic form (táimse) or the analytic form (tá mése) equally.
One interesting thing you will see, that you don't see in many languages in Europe, is the similarity between the ending and the pronoun.
Είμαι εγώ: με
This is because most languages have this strange word εγώ that does not resemble με. Irish (and other Celtic languages) just use the equivalent of με as it has lost the nominative/accusative contrast completely.
They are pronounced the same but used to be pronounced differently many thousands of years ago -- that's why there are separate letters.
Nowadays, the spelling follows etymological/historical rules, so in many cases, you have to learn which letter to use.
It is very much like "ee/ea" in English -- "meet" and "meat" sound exactly the same for nearly all English speakers, though a few dialects still preserve a difference there. All others simply have to learn which word takes which spelling.
A lot of English words come from Greek or are related to it, so this old spelling has two effects.
Firstly it makes it easier to recognize the connections, since there is a strict relationship. Often the shape of the letter, especially the capitals is the clue, so Ι = I, Β = B, Υ = Y, Ε = E. They don't all work, eg Η = E.
Secondly, if you can recognise the English word it will be a good clue as to which Greek letter to use when there is a choice.
In the "body" of the word, you just have to learn the spelling.
It is like "ea" versus "ee" in English. Why do we write "leaf" but "beef", "seat" but "street", and so on when they sound identical?
There are historical reasons (the sounds used to be different), and this is true for Greek as well -- but in terms of the modern language, you just have to learn the spelling, because both Greek and English use a historical spelling.
In the endings, it's a bit easier, as grammar (not just history/etymology) can help you.
For example, a word ending in an /o/ sound will end in -ω if it's a verb but in -ο if it's a noun. (A bit simplified but mostly true.) Similarly, a noun eing in an /i/ sound will end in -η if it's feminine but in -ι if it's neuter. (Again, a bit simplified.)
Greek has only one present form and this sentence can be translated as either...
''I drink water and eat bread."
or "I am drinking water and eating bread." etc
Both forms are accepted for this sentence. If it was not accepted there might have been another problem since we can't see what you wrote you should send us a screenshot so we can understand what the problem was.
Let me see if I understand.
We moderators cannot see your sentence or the kind of exercise it was. You had a listening exercise where the sentence was said in GReek and you had a choice of words (in Greek) but when you chose the right words it said the answer should have been in English?
That seems to be a technical glitch and it's the first we've heard. You need to report it to:https://support.duolingo.com/hc/en-us
Did this happen today? Did you report it as "My sentence should be accepted.? That might help.