Jogurt mi je kiseo, dok je umak blag i lagan.

Mi is the unstressed dative form of ja (I / me). In this sentence, it means something like to me.

So Jogurt mi je kiseo literally feels like:

• The yogurt is sour to me

In natural English, this is often just translated as My yogurt is sour or The yogurt tastes sour to me, depending on context.

It does not show possession directly. It shows the speaker’s personal experience or judgment.

Because there are two clauses, and each clause has its own form of to be:

• Jogurt mi je kiseo
• dok je umak blag i lagan

Here, je is the 3rd person singular present form of biti (to be).

So the structure is:

• Jogurt ... je kiseo = The yogurt is sour
• umak ... je blag i lagan = the sauce is mild and light

Croatian usually keeps the verb in each clause, even when English might sometimes feel more flexible.

This is because je is a clitic, and Croatian clitics usually go in the second position of the clause.

In the clause:

• dok je umak blag i lagan

the first element is dok, so the clitic je comes right after it.

That is why dok je umak... sounds natural, while dok umak je... does not.

The same thing happens in the first clause with mi je:

• Jogurt mi je kiseo

Both mi and je are clitics, so they appear in the normal Croatian clitic position.

They agree with the nouns they describe.

Here:

• jogurt is masculine singular
• umak is masculine singular

So the adjectives also appear in the masculine singular nominative form:

• kiseo
• blag
• lagan

If the noun changed, the adjective would change too. For example:

• Juha je blaga. — The soup is mild.
  (juha is feminine, so blag becomes blaga)

• Piće je kiselo. — The drink is sour.
  (piće is neuter, so kiseo becomes kiselo)

Because this sentence uses a predicate adjective, not an adjective directly attached to the noun.

Compare:

• kiseo jogurt = sour yogurt
• Jogurt je kiseo = The yogurt is sour

In your sentence, the speaker is making a statement about the yogurt, so Croatian uses:

• noun + je
  • adjective

That is the normal pattern for X is Y sentences.

Here dok means while or whereas.

It introduces a contrast:

• Jogurt mi je kiseo, dok je umak blag i lagan.
• The yogurt is sour, whereas the sauce is mild and light.

So this dok is not mainly about time. It is being used contrastively.

In other contexts, dok can also mean while in a time sense, but here the important idea is comparison or contrast.

Yes. That is completely grammatical.

• Jogurt je kiseo. = The yogurt is sour.

Adding mi makes it more personal:

• Jogurt mi je kiseo. = The yogurt is sour to me / My yogurt is sour / It tastes sour to me.

So:

• without mi = more neutral, objective statement
• with mi = more personal impression or relevance to the speaker

Blag often means:

• mild
• gentle
• not strong
• sometimes not spicy

With food, it usually means the flavor is not intense or harsh.

So umak blag is usually:

• a mild sauce

Depending on context, it could suggest:

• not spicy
• not acidic
• not heavily seasoned

It is usually more positive than English bland, although in some situations the idea can overlap.

With food, lagan usually means light.

That can mean different but related things:

• not heavy
• easy to digest
• not rich or greasy
• light in texture or overall effect

So umak blag i lagan suggests a sauce that is:

• mild in flavor
• light rather than heavy

It does not necessarily mean physically lightweight in the literal sense.

Because Croatian has no articles.

So Croatian does not have separate words matching English:

• a
• an
• the

That means:

• jogurt can mean yogurt, a yogurt, or the yogurt
• umak can mean sauce, a sauce, or the sauce

The exact meaning is understood from context.

In this sentence, English naturally uses the:

• The yogurt is sour, whereas the sauce is mild and light.

But Croatian does not need an article to express that.