English can be really confusing when it comes to singular and plural nouns. We have words from so many different languages that still follow their native language's grammatical rules that we can't easily say "This is a singular noun and this is how you make it plural."
I mean, look at octopus (octopi, from Latin, or octopuses, modern bastardization) or child (children, what the fuck?), goose (geese) and moose (meese? damn, moose). English makes no real sense that way and every day that I study Latvian, I wonder how the hell I can manage to speak English at all.
Latvian is much simpler. Oh, to every rule there is an exception, but overall, it's pretty simple. Other than those evil 6th declension buggers (which are few and getting fewer every year), you can identify whether a noun is grammatically masculine or feminine and what declension it may be in at a glance.
Nouns in the 1st, 2nd and 3rd declensions are generally masculine.
Nouns in the 4th, 5th and 6th declensions are generally feminine.
Exceptions are words like puika (boy) which comes from a Finn-Ugric language, though I'm not sure which. Puika is masculine even though it is in the 4th declension. Still, it declines like any other 4th declension noun.
Nouns ending in -o , like radio, do not belong to any declension and are considered indeclinable. So they remain the same no matter what case they are in.
To form the plural of any 1st, 2nd or 3rd declension noun , whose singular form ends in -s , -š , -is or -us , change the ending to -i.
- 1st: koks (tree) -> koki (trees)
- 2nd: kuģis (ship) -> kuģi (ships)
- 3rd: tirgus (market) -> tirgi (markets)
To form the plural of any 4th or 5th declension noun , whose singular form ends in -a , or -e , add an -s to the end.
- 4th: diena (day) -> dienas (days)
- 5th: upe (river) -> upes (rivers)
To form the plural of 6th declension nouns (those evil buggers), whose singular form ends in -s , insert an -i before the final -s.
- 6th: govs (cow) -> govis (cows)