Ako mikser ne radi, umutit ću vrhnje ručno.

Because ako means if, so the sentence has:

• an if-clause: Ako mikser ne radi = If the mixer doesn’t work
• a main clause: umutit ću vrhnje ručno = I’ll whip the cream by hand

This is a very common Croatian pattern, just like English If X happens, I’ll do Y.

After ako (if), Croatian normally uses the present tense to talk about a future condition, just like English often does:

• Ako mikser ne radi... = If the mixer doesn’t work...

So even though the whole situation is about the future, the verb in the if-clause is present.

Here radi comes from raditi, and with machines it means to work, to function, or to be operating.

So:

• Mikser radi. = The mixer works / is working.
• Mikser ne radi. = The mixer doesn’t work / isn’t working.

It is not about the mixer doing work in a human sense; it simply means the appliance functions.

This is the future tense in Croatian.

The infinitive is umutiti = to whip / to beat up.

In standard Croatian, when the future auxiliary ću comes after the infinitive, the final -i of the infinitive drops:

• umutiti + ću → umutit ću

So umutit ću means I will whip.

You can also see a structure like:

• Ja ću umutiti vrhnje.

That is also correct. But in your sentence, the verb comes first in the main clause, so the form is umutit ću.

Ću is the 1st person singular future auxiliary, meaning will in I will.

So:

• ću = I will
• ćeš = you will
• će = he/she/it will
• ćemo = we will
• ćete = you (plural) will
• će = they will

In this sentence, umutit ću literally corresponds to will whip I, but in natural English we say I’ll whip.

Croatian often drops subject pronouns when they are clear from the verb form.

Here, ću already tells you the subject is I, so ja is unnecessary.

• umutit ću = I will whip

You could say ja ću umutiti vrhnje ručno if you want extra emphasis on I, but it is not needed in a neutral sentence.

This is about aspect, which is very important in Croatian.

• mutiti = imperfective: the action in progress, repeated, or not viewed as completed
• umutiti = perfective: the action as a completed whole

In this sentence, umutit ću is perfective because the speaker means I’ll whip it up as a completed action.

So umutiti fits better than mutiti here.

Vrhnje is the direct object, so it is in the accusative case.

However, vrhnje is a neuter singular noun, and for many neuter nouns the nominative and accusative forms are the same.

So:

• nominative: vrhnje
• accusative: vrhnje

That is why it looks unchanged even though it is the object of the verb.

Vrhnje means cream.

Depending on context, it can refer to different kinds of cream, but in this sentence, because of umutit (to whip), the meaning is naturally whipping cream or simply cream to be whipped.

So the sentence is understood as something like:

• If the mixer doesn’t work, I’ll whip the cream by hand.

Ručno means by hand or manually.

It is an adverb, so it describes how the action is done:

• umutit ću vrhnje ručno = I’ll whip the cream by hand

It contrasts with using a machine such as mikser.

Yes, you often can, but the nuance is slightly different.

• ručno = manually / by hand in a general sense
• rukom = with the hand more literally

So both can work, but ručno sounds very natural when contrasting manual work with a machine:

• Ako mikser ne radi, umutit ću vrhnje ručno.

Because the sentence begins with a subordinate if-clause:

• Ako mikser ne radi, ...

When this clause comes first, Croatian normally separates it from the main clause with a comma. This is very similar to English:

• If the mixer doesn’t work, I’ll whip the cream by hand.

Croatian has no articles, so there is no direct equivalent of English a/an/the.

That means:

• mikser can mean a mixer or the mixer
• vrhnje can mean cream or the cream

The exact meaning comes from context, not from an article.

A rough guide:

• ću sounds approximately like tyoo, but with a softer t sound than English
• vrhnje is tricky for English speakers; roughly VERH-nyeh or vrh-nyeh

A few useful points:

• ć is a soft Croatian sound, not exactly the same as English ch
• nj is like the ny in canyon

So vrhnje has that ny sound at the end: vrh-nye.