Odakle znaš da će do jutra biti samo dva stupnja?

Yes. Odakle literally means from where.

But in sentences like Odakle znaš...?, Croatian often uses it idiomatically in the sense of:

• How do you know...?
• Where did you get that information from?

So the idea is not really about physical location. It is asking about the source of someone’s knowledge.

Znaš is the 2nd person singular present tense of znati (to know).

So:

• ja znam = I know
• ti znaš = you know
• on/ona/ono zna = he/she/it knows

Here znaš means you know.

Croatian usually does not need the subject pronoun ti, because the verb ending already shows who the subject is. So Odakle ti znaš... is possible for emphasis, but normally just Odakle znaš... is enough.

After verbs like znati (to know), Croatian often uses da to introduce a subordinate clause.

So:

• Znam da... = I know that...
• Znaš da... = You know that...

In this sentence:

• Odakle znaš da će do jutra biti samo dva stupnja?

the da introduces the content of what the person supposedly knows: that by morning it will be only two degrees.

This is very normal Croatian structure.

Croatian future tense uses the auxiliary ću, ćeš, će, ćemo, ćete, će plus the infinitive.

With many verbs in simple main clauses, you often see forms like:

• ići ću = I will go
• reći će = he/she will say

But in a clause introduced by da, the auxiliary normally comes before the infinitive:

• da će biti = that it will be

So here:

• da će do jutra biti...

is exactly what you would expect in a subordinate clause.

Croatian does not need a dummy subject like English it in weather or general-condition statements.

English says:

• It will be cold
• It will be two degrees

Croatian can simply say:

• Bit će hladno
• Bit će dva stupnja

So in this sentence, biti is used impersonally. There is no separate word corresponding to English it.

Do jutra means until morning or by morning, depending on context.

Here it most naturally means:

• by morning
• by the time morning comes

The preposition do requires the genitive case, which is why you get:

• jutro = morning
• do jutra = until/by morning

So this is a standard case pattern:

• do večeri = by evening
• do kraja = until the end
• do jutra = by morning

This is because Croatian numbers affect the form of the noun.

With 2, 3, 4, nouns usually take a special form that often looks like the genitive singular:

• dva stupnja = two degrees
• tri stupnja = three degrees
• četiri stupnja = four degrees

With 5 and above, you normally use the genitive plural:

• pet stupnjeva = five degrees
• šest stupnjeva = six degrees

And stupnjevi is the regular nominative plural form, so it would not be used after dva here.

Grammatically, it is a form that looks like genitive singular, but after numbers like dva, tri, četiri, Croatian uses this special numeral pattern.

So in meaning, dva stupnja is clearly plural: two degrees.

This is one of those places where Croatian grammar does not match English very neatly. The best approach is to learn the pattern:

• jedan stupanj
• dva / tri / četiri stupnja
• pet+ stupnjeva

Samo means only.

So:

• samo dva stupnja = only two degrees

It emphasizes that the temperature will be low.

Its position is natural here because it directly highlights the number:

• biti samo dva stupnja = be only two degrees

Yes. Znaš is singular informal you.

If you were speaking politely or to more than one person, you would use znate:

• Odakle znate da će do jutra biti samo dva stupnja?

So:

• znaš = you know (informal singular)
• znate = you know (formal singular or plural)

Croatian word order is fairly flexible, but not completely free.

The given sentence:

• Odakle znaš da će do jutra biti samo dva stupnja?

is very natural and neutral.

You could move some parts for emphasis, for example:

• Odakle znaš da će biti samo dva stupnja do jutra?

That is possible, but it sounds a bit different in focus and is less neutral.

The original word order is the best one to learn first:

• question word first: Odakle
• main verb: znaš
• subordinate clause: da će do jutra biti samo dva stupnja

In everyday Croatian, yes, usually.

When people talk about the weather and say:

• bit će dva stupnja

they normally mean two degrees Celsius, unless another scale is specifically mentioned.

If needed, someone could say:

• dva stupnja Celzija

But in ordinary conversation, stupnja by itself is usually enough.