Quantities (some water, many people) can be expressed in several ways in Croatian.
It's important to make first a distinction between countable nouns (apple, car, sister...) and uncountable nouns (air, salt, time...). In English, you can e.g. use few with countable nouns (few cars) but not with uncountable ones (no few salt but rather little salt).
In Croatian, if you want to express some quantity of an uncountable noun, you can use just the noun in genitive case (instead of any other case it should be put to, according to its role in sentence):
Imamo kruha. We have some bread.
If you want to express the same thing, but for countable nouns, you should use the genitive case in plural:
Imamo jabuka. We have some apples.
Note how English uses the noun in singular in the first sentence, but the noun in plural in the second sentence, exactly corresponding to the Croatian forms (except for the genitive case, of course).
Before such nouns in genitive, you can place one of the following adverbs of quantity:
malo a bit / few|
dosta quite a few
premalo too little / few|
previše too much / many
nedovoljno not enough
| much / many|
/ a lot of
The word puno is more colloquial than mnogo. These words can be used with both countable and uncountable nouns, but the nouns should be in genitive singular (if uncountable) or plural (if countable):
Imamo premalo kruha. We have too little bread.
Imamo puno jabuka. We have a lot of apples.
Such quantities — if only a genitive is used and if a quantity adverb is used — act as being neuter singular. Verbs and other words must be set accordingly:
Puno ljudi je došlo. A lot of people came.
This is quite unlike English!
You can use personal pronouns instead of nouns (again in G-pl, there's no change in grammar of other parts):
Puno ih je došlo. A lot of them came.
As you can see, you can use the short forms of pers. pronouns, but they must be at the second position.
Unlike English, words like puno a lot can be separated from the nouns, without much change in meaning:
Puno je ljudi došlo.
Došlo ih je puno.
It's obvious that puno refers to ljudi and ih, since the noun and the pronoun are in G-pl.
There are three words reused to express small and indefinite quantities, like English some:
nešto some (adverb)
neki one, some (countable, adjective)
koji few (countable, adjective)
All three words have other functions as well. The word nešto does not change in case when in this role, and behaves like malo, but it emphasizes that the quantity is small and not really known. It can be used with both countable and uncountable nouns:
Imam nešto ml
ijeka. (G) I have some milk.
Imam nešto limuna. (G-pl) I have few lemons.
Words neki and koji are of course adjectives (koji has special shortened forms as well). They are used with countable nouns only and don't change the case of the following noun, they don't use the G-pl. For example:
Imam koju jabuku. (A) I have few apples.
Imam neku jabuku negdje. I have an apple somewhere.
The word neki stands for a completely indeterminate, single thing of some kind, while koji stands for a unknown, but likely small number of things of some kind. Here singular is used with koji, but the meaning is plural.
Next, we're able to express existence of any/certain amount of something (e.g. there are some apples). While English uses dummy there, Croatian uses the verb imati have in the 'impersonal' form (without any subject, in the 3rd person singular). The nouns are again in G-pl for countable nouns, G-sg for uncountables:
Ima° jabuka. (G-pl) There are some apples.
Ima° vode. (G) There is some water.
(Pay attention that countable nouns always use G-pl in any constructs involving quantities. The only exception is with numbers 2-4, neki and koji.)
Since these sentences are impersonal (literally: it has some apples), as with any impersonal sentence, the past tense forms must be in neuter singular. However, such expressions are quite special: in the past and future tenses, you have to use the verb biti (je² +) be instead of imati:
Bilo je jabuka. There were some apples.
Bilo je vode. There was some water.
Bit će krvi. There will be blood.
Negative sentences in present use nema in the present tense, but again the verb biti in other tenses:
Nema° vode. There's no water.
Nije bilo vode. There was no water.
Neće biti vode. There will be no water.
This includes a very common phrase, corresponding to English no problem (enter it into Google™):
Nema° problema. There are no problems.
Croatian cannot negate nouns themselves (as English no or German kein): verbs must be negated in such sentences. Such sentences are no way limited to indefinite amounts ("some") — they can express existence (or negation of existence) of any quantity:
Bilo je previše problema. There were too many problems.
Ima dosta ljudi. There are quite a few people.
You can also express that there's no someone:
Nema° Gorana. There's no Goran.
Unlike English, such constructs are often used with personal and indefinite pronouns:
Nema° te. You're not here.
Nema° je. She's not here.
Nema° ih. They're not here.
Nije te bilo. You weren't here.
Nema° nikog. Nobody's here.
The pronouns te², je² and ih² are, of course, in the genitive case. In the last sentence, the pronoun netko must be in its negated form nitko, since the main verb — nema — is negative.
The Croatian sentences don't really say here (they say 'there's no you', literally, 'it hasn't you', and so on); you can add where somebody is not found:
Nema° ih u uredu. They're not in the office.
Nema° nikog u sobi. Nobody's in the room.
You can also use such existential expressions to say that there's someone, but it's less often used.
If you want to express existence of indefinite quantity at some location, the neutral expression is:
Bilo je vode u boci. There was some water in the bottle.
The following expression specifically talks about what's in the bottle, literally in bottle is water (the noun voda is in the nominative case):
U boci je voda. (N) There is water in the bottle.
If we change the word order, there's a subtle change in meaning: we're no more talking about the bottle, we're talking about the water, and where it is:
Voda je u boci. The water is in the bottle.
The word order in Croatian is mostly such that what we're talking about comes first, and the information we want to give comes later. This is also a way of expressing definiteness, since Croatian has no articles. Another example:
Ključevi su u ladici. The keys are in the drawer.
U ladici su (neki) ključevi. There are (some) keys in the drawer.
In the first sentence, we're talking about where the keys are, but in the second, we're discussing what's in the drawer, what we have found.
Finally, Croatian really doesn't express strict difference like Spanish no hay (doesn't exist) vs. no está (not here), but if you explicitly say where, it's obvious that something exists but it's just not here:
Goran nije ovdje. Goran isn't here.