Du kan stole på mig, hvis du har brug for hjælp.

Du means you and is the normal singular form used in everyday Danish.

A few useful notes:

• du is used for one person
• in modern Danish, du is also used in most situations where English might still use a more formal tone
• the old formal form De exists, but it is much less common and sounds quite formal or old-fashioned in many contexts

So Du kan stole på mig ... is a natural, normal way to speak to one person.

Because after a modal verb like kan (can), Danish uses the infinitive form of the next verb.

So:

• kan = can
• stole = trust
• not stoler

Compare:

• Jeg stoler på dig. = I trust you.
• Jeg kan stole på dig. = I can trust you.

In the first sentence, stoler is the present tense. In the second, stole is the infinitive because it follows kan.

Stole på is a fixed verb expression in Danish. It means to trust or to rely on someone.

You should learn it as one unit:

• at stole på nogen = to trust someone

The preposition på is required here. Danish often uses prepositions in places where English does not, or uses a different one.

Examples:

• Jeg stoler på dig. = I trust you.
• Hun stoler ikke på ham. = She does not trust him.

So stole på is not just stole by itself in this meaning.

Because mig is the object form of jeg.

Danish pronouns change form depending on their role:

• jeg = I
• mig = me

After a preposition like på, you use the object form:

• på mig = on me / me

So:

• Jeg stoler på dig. = I trust you.
• Du kan stole på mig. = You can trust me.

Not på jeg.

Here hvis means if.

It introduces a condition:

• hvis du har brug for hjælp = if you need help

A useful distinction:

• hvis = if
• om = whether / if

Examples:

• Jeg hjælper dig, hvis du spørger. = I’ll help you if you ask.
• Jeg ved ikke, om han kommer. = I don’t know whether he is coming.

So in this sentence, hvis is the correct word because it introduces a condition.

At have brug for is a very common Danish expression meaning to need.

Literally, it is something like to have use for, but you should treat it as a normal expression for need.

So:

• du har brug for hjælp = you need help

Danish also has at behøve (to need), but have brug for is extremely common and often sounds very natural in everyday speech.

Compare:

• Jeg har brug for hjælp. = I need help.
• Jeg behøver hjælp. = I need help.

Both are possible, but har brug for is especially common.

Because hjælp here is being used as an uncountable noun, like help in English.

So Danish says:

• brug for hjælp = need help

Not:

• brug for en hjælp

That would sound wrong in this context.

A few comparisons:

• Jeg har brug for hjælp. = I need help.
• Han gav mig hjælp. = He gave me help.

If you want something more specific, Danish may use other wording, but in this sentence plain hjælp is exactly what you want.

Because hvis du har brug for hjælp is a subordinate clause, and Danish subordinate clauses usually keep the order:

conjunction + subject + finite verb + rest

So:

• hvis = conjunction
• du = subject
• har = finite verb

That gives:

• hvis du har brug for hjælp

This is different from main clauses, where Danish usually follows the verb-second rule.

For example:

• Du kan stole på mig.
  Here kan is in second position.

If you put the hvis-clause first, the main clause changes:

• Hvis du har brug for hjælp, kan du stole på mig.

Notice kan du, not du kan, because the sentence still keeps verb-second order in the main clause.

The comma before hvis can depend on which Danish comma system is being used.

You may see:

• Du kan stole på mig, hvis du har brug for hjælp.
• Du kan stole på mig hvis du har brug for hjælp.

Both can occur.

In traditional Danish comma usage, a comma often appears before subordinate clauses like the one introduced by hvis. In newer usage, that comma is often left out.

So the version you were given is perfectly normal, but you may also see the sentence without the comma.

Yes. That is also correct and very natural.

The difference is mostly about emphasis:

• Du kan stole på mig, hvis du har brug for hjælp.
  Starts with the reassurance: You can trust me
• Hvis du har brug for hjælp, kan du stole på mig.
  Starts with the condition: If you need help

When the hvis-clause comes first, the main clause uses inversion:

• kan du not
• du kan

That happens because Danish main clauses normally keep the finite verb in second position.

It can often mean both, depending on context.

At stole på nogen usually means:

• to trust someone
• to rely on someone

So in a sentence like this, English translations such as these can all fit:

• You can trust me
• You can rely on me
• You can count on me

The exact English choice depends on tone and context, but the Danish is completely natural for offering support.