This varies, depending on the ending of the noun. These are the basic rules to follow for definite articles, singular, nominative/accusative form:
-consonant: + "-ul"; e.g. sat - satul (village - the village)
-i: + "-ul"; e.g. război - războiul (war - the war)
-u: + "-l"; e.g. ou - oul (egg - the egg)
-e: + "-le"; e.g. rege - regele (king - the king)
-ă: There are very few masculine nouns ending in "ă":
tată - tatăl (father - the father)
papă - papa (pope - the pope)
popă - popa (priest - the priest)
pașă - pașa (Ottoman rank)
-ă: replaced by "-a"; e.g. masă - masa (table - the table)
-e: replaced by "-ea"
*; e.g. lumânare - lumânarea (candle - the candle)
-ie: replaced by "-ia"
**; e.g. farfurie - farfuria (plate - the plate) ; cheie - cheia (key - the key)
-a: + "-ua"; e.g. sarma - sarmaua (popular Romanian food)
-ea: + "-ua"; e.g. lalea - laleaua (tulip - the tulip)
-i : + "-ua"; e.g. zi - ziua (day - the day)
* be careful that this is a diphthong and no additional syllable occurs
**this may be misleading, as "ie" can be both a diphthong or two separate vowels. The same relation is kept with "ia":
/far.fu.ri.e/ - /far.fu.ri.a/
/ke.je/ - /ke.ja/
(I am not very good with IPA, but I hope that is clear enough for people to get the idea.)
Note that neutral nouns just behave like masculine nouns in the singular (and feminine nouns in the plural) so they are also covered here.
Disclaimer: I am a native speaker, but there might be some weird, obscure exceptions that I can't think of right now.
All of the definite articles in romance languages were developed from other kinds of articles, romanian was the only one to use an article that was also used in latin after the noun. Its position probably made them turn it into a suffix in time.
That way, the latin "romanu illu" (meaning "that romanian" in latin) became românul (meaning "the romanian" in romanian). You can read more about it here: