"egy fiú"

Translation: a boy

[AmyAbramcz]

How can we tell when egy is used as 'one' and when it's used as 'the'

[KuroKasai]

'Egy' means 'a(n)', not 'the'

[Lomochibi]

You will know when 'egy' is used as 'one' and when as 'a(n)' from context, and in some cases intonation (in speach).

[MrtonPolgr]

And anyway, it clearly implies the amount to be one. The difference isn't huge, it's just a matter of emphasis really. (just like in German or Romanian)

[curiousminds]

Why 'egy' and not 'a'/'az'?

[Lomochibi]

'Egy' is an indefinite article, 'az' is a definite article, similar to 'a(n)' and 'the' in English.