o bere - a beer (fem sg indef)
berea - the beer (fem sg def.)
beri- beers (fem pl indef)
berile - the beers (fem pl def)
O sticlă de bere.
In Romanian, for most nouns (the ones that are countable) we can find the gender by counting them and observing the pattern for singular and plural forms of the numerals associated with them.
There are 3 genders, so there are 3 patterns:
MASCULINE: UN - DOI (singular - plural)
- Examples: un bărbat - doi bărbați (man), un copac - doi copaci (tree), un perete - doi pereți (wall)
FEMININE: O - DOUĂ (singular - plural)
- Examples: o femeie - două femei (woman), o floare - două flori (flower), o casă - două case (house)
NEUTER: UN - DOUĂ (singular - plural)
- Examples: un picior - două picioare (leg), un tablou - două tablouri (painting), un peisaj - două peisaje (landscape)
For uncountable nouns it is more complicated. There are many patterns and exceptions. Best thing is to check with a dictionary (for example https://dexonline.ro). Right after the noun you will see either s.m, s.f. or s.n. abbreviations for masculine, feminine and neuter genders.
i find it funny when i ask my girlfriend the gender of a word, she tells me to count them to find out and i get it wrong because i don't know the gender.