[#DuolingoForumGems originally posted on 2020-02-15 on the Duolingo Hungarian for  English speakers forum by jzsuzsi ] 

Using egy

There have been questions about "egy", the indefinite article. When do we use it? When do we omit it? When is it optional?

Well, including or dropping "egy" creates a small difference in meaning. But since Duolingo is translation-based, as long as the English translation stays the same, these nuances in meaning are kind of invisible. 

Uncountable method

vvsey has a saying:  In Hungarian, treat all of the nouns as uncountable.  Let's look at some English uncountable nouns:

furniture - bútor
a piece of furniture - egy bútor
bread - kenyér
a loaf of bread - egy kenyér
beer - sör
a (bottle of) beer - egy sör

Ok, then apply this logic to nouns that are countable in English:
one apple - egy alma
apples, but the quantity in unknown/ not important - alma

a flower - egy virág
flowers, but the quantity in unknown/ not important - virág

You can point at one apple and say  "Ez egy alma."
You can point at a crate full of apples and ask "Ez alma?"  

According to the book

Let's see, what Judit's favourite book (Practical Hungarian Grammar by Szita & Görbe, page 118) says about articles:

Both Hungarian and English use an indefinite article in the following cases:

1.1. preceding new information (a noun that has not been introduced yet)
Találtam egy macskát. I found a cat. 
Láttam egy zsiráfot az állatkertben.  I saw a giraffe at the zoo.

1.2. When defining quantity
Egy csésze teát kérek. I'd like a cup of tea.
Teszek egy kevés paprikát a levesbe. I put a little paprika in the soup.

When do we not use an article?
In almost all other cases when English uses an indefinite article, Hungarian does not. 

2.1.1. Preceding names for categories, e.g. professions.
Judit ápolónő. Judit is a nurse.
A veréb madár.  A sparrow is a bird.
A villamos közlekedési eszköz. A tram is a vehicle. 

2.1.2. With undefined quantity.
Veszek kenyeret. I'll buy some bread.
Kérsz kávét? Would you like some coffee?

2.1.3. In expressions that answer the question milyen? (what kind of?)
Ez komoly probléma.  This is a serious problem.
A buli jó ötlet. The party is a good idea.
Szép képet festesz. You are painting a nice picture. 

2.2. In dates, English uses a definite article when Hungarian does not use any article. 
Ma augusztus 12. van. Today is the 12th of August.

2.3. Similarly to English, we do not use any article preceding names for persons, countries in singular, cities and continents. 
Brigitta beteg. Brigit is ill.
Kanada északi ország. Canada is a northern country.
But: Az USA-ba utazom. I am travelling to the USA. 

Other comments

Languages change. The creators of the Hungarian course used a book published in 1990, and in this book, the first chapter has "Ez táska." "Az autó." as example sentences. But nowadays native Hungarians complain that this sounds wrong, they would say "Ez egy táska." "Az egy autó." instead. 

For 2.1.3 above, I would say it is also acceptable to use these sentences with egy.
Ez egy komoly probléma.  Egy szép képet festesz.

I asked others too, AndrsBrny said:
I think in general "egy" vs. nothing makes a slight distinction in whether you're really referring to an instance of the thing you're talking about or an unspecified amount or mass of that thing (although the distinction is probably not always quite this clear).

For Éva bought flowers., it's most natural to say Éva virágot vett. or Virágot vett Éva., and the plural is actually odd in Hungarian. And of course saying Egy virágot vett Éva. would mean she bought a single flower. So with words like virág, the unmarked singular is actually the least specific form, just referring to the thing itself.

vvsey said:
Without "egy", it is like describing a characteristic. With "egy", it is like an instance of a given thing.

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peter.kristof.hu also had a post about articles:
 https://forum.duolingo.com/comment/30871735
https://magyarbagoly.blogspot.com/2022/02/the-use-of-articles-in-hungarian.html

see also here: 
sentence discussion
The old building is a bank.
A régi épület egy bank.

https://forum.duolingo.com/comment/17015029 = https://archive.ph/yPR9x

Comment by weitzhandler

Why isn't a régi épület bank a proper answer?

Comment by vvsey

It is fine but it does sound a bit more natural with "egy".

Comment Tielbert

And what if i omit egy? How does it change the sense nuance?

Comment by vvsey

I am not sure there is even a nuance. But if I want to push it and find a tiny bit of difference, I would say "egy bank" identifies it as "a bank", while if we just say "bank", then it ALMOST sounds like an adjective. So, it would refer to the functionality of the building, as a characteristic of it. What it is used for. What it serves as.

"A régi épület egy bank." - The old building is a bank.

"A régi épület bank." - The old building serves as a bank. It functions as a bank.

Does this make sense?

Let's assume that the building has served as a bank for a hundred years. But various banks came and went during the years. So it has always served as a bank in the last hundred years, but it changed owners/tenants several times. And of course it is a bank now, as well.
"Az elmúlt száz évben mindig bank volt, és most is (egy) bank."

Maybe the same tiny difference exists with professions, and probably many other sentences:

"Én tanár vagyok." - I serve/work/function as a teacher. My profession is teacher.

"Én egy tanár vagyok." - I am a member of the group of people who function as teachers. I am an instance, a specimen, of the definition of "teacher".

But this distinction may not actually exist at all, or not consciously. You may be better off just learning something like "we do not use 'egy' with professions". If you know a little Spanish, you may lean on that knowledge when deciding when to use "egy", as there are some similarities between the two languages in this regard. And also in the use of definite articles.

sentence discussion
In front of the big airport a red bus is waiting.
A nagy repülőtér előtt egy piros busz vár.

https://forum.duolingo.com/comment/20324526 = https://archive.ph/BXRbC

Comment by AmineHadji1

Is there a reason "A nagy repülőtér előtt piros busz vár" (without egy) is not accepted? I thought the egy was almost always optional.

Comment by MrtonPolgr

Your sentence is meaningful and legit, BUT... egy never was almost always optional, I wouldn't even say it has ever been optional. It's just certain categories by which we use articles are rather fluid.
My rule of thumb is: if you don't use "egy", you don't introduce a well-defined instance, you just create some disposable stuff of flexible amount that you won't refer back to. (Think of a sentence like "I've found some money" - here "some money" could very well translate to "pénz" ("pénzt" in the sentence, because accusative) without any articles. Now imagine using "some" similarly for countable nouns too...)
So, what about your sentence? You actually didn't risk a lot here because you have put "piros busz" in the focus position and you sorta defined the noun by using an adjective anyway. It may sound a tiny little bit pretentious though, without the article (feels like "There is some red bus waiting in front of the big airport")