Hvis det ikke lykkes hende at logge ind i aften, prøver hun igen i morgen.

Hvis means if and introduces a conditional clause.

So:

• Hvis det ikke lykkes hende at logge ind i aften = If she doesn’t manage to log in tonight
• prøver hun igen i morgen = she tries again tomorrow

A very common pattern in Danish is:

• Hvis + clause, main clause

Just like in English:

• If it rains, we stay home.

Lykkes means something like succeed, work out, or manage to happen successfully.

In this sentence, det lykkes hende literally means:

• it succeeds for her

Natural English:

• she succeeds
• she manages
• it works for her

A key thing for English speakers is that Danish often uses lykkes in a structure where the thing is the grammatical subject:

• Det lykkes hende at logge ind
  = She manages to log in
  literally: It succeeds for her to log in

So lykkes does not behave exactly like English succeed in all cases.

Because lykkes is commonly used with a dummy subject det in expressions like this.

• Det lykkes hende at logge ind
• literally: It succeeds for her to log in

This is a normal Danish construction. English usually prefers:

• She manages to log in
• She succeeds in logging in

So det here does not really mean a concrete it. It is just part of the structure.

Because hende is the object form of hun.

• hun = she (subject form)
• hende = her (object form)

In this sentence:

• det lykkes hende
  literally: it succeeds for her

Since she is not the subject of lykkes, Danish uses hende, not hun.

Compare:

• Hun prøver igen i morgen. = She tries again tomorrow.
• Det lykkes hende at logge ind. = She manages to log in.

In the first sentence, she is the subject, so hun is used. In the second, she is part of the for her idea, so hende is used.

At logge ind means to log in.

• logge ind = log in
• at logge ind = to log in

The word at is the infinitive marker, like English to before a verb.

Examples:

• at spise = to eat
• at læse = to read
• at logge ind = to log in

So in this sentence:

• det lykkes hende at logge ind
• she manages to log in

Because logge ind is a Danish verb phrase, similar to English log in.

Some Danish verbs are made up of a verb + particle:

• gå ud = go out
• komme ind = come in
• skrive ned = write down
• logge ind = log in

In the infinitive, they often appear together:

• at logge ind

In some other sentence patterns, the particle can be separated depending on the structure, but here at logge ind is the normal infinitive phrase.

This is because Danish follows the V2 rule in main clauses: the finite verb usually comes in second position.

The sentence begins with a subordinate clause:

• Hvis det ikke lykkes hende at logge ind i aften

After that, the main clause starts. Since the if-clause takes the first position overall, the finite verb in the main clause comes next:

• prøver hun igen i morgen

not:

• hun prøver igen i morgen

This is very typical Danish word order.

Compare:

• Hun prøver igen i morgen.
• Hvis det ikke lykkes, prøver hun igen i morgen.

Same main clause, but after the initial hvis clause, the verb must come before the subject.

Ikke means not.

In this subordinate clause, it comes before the finite verb:

• Hvis det ikke lykkes hende ...

This is normal Danish word order in subordinate clauses. A useful contrast is:

• Main clause: Hun prøver ikke igen.
• Subordinate clause: ... hvis hun ikke prøver igen

So English speakers often need to notice this difference:

• main clauses and subordinate clauses do not place adverbs like ikke in the same position.

In these fixed time expressions, Danish uses i where English often uses no preposition.

• i aften = tonight
• i morgen = tomorrow

You should learn them as common expressions.

Other examples:

• i dag = today
• i nat = tonight / at night
• i går = yesterday

So even though English says just tonight and tomorrow, Danish often uses i in these expressions.

Danish often uses the present tense for future meaning when the time is clear from context.

Here:

• Hvis det ikke lykkes ... i aften = future reference
• prøver hun igen i morgen = also future reference

Because i aften and i morgen clearly show the time, Danish does not need a special future form.

This is similar to English in sentences like:

• If it doesn’t work tonight, she tries again tomorrow.

In natural English, we would more often say:

• If she can’t log in tonight, she’ll try again tomorrow.

But in Danish, present tense for future situations is very common.

Yes, but it would mean something slightly different.

• Hvis det ikke lykkes hende at logge ind i aften ... = If she doesn’t manage to log in tonight ... This focuses on whether the attempt is successful.

• Hvis hun ikke kan logge ind i aften ... = If she can’t log in tonight ... This focuses more on ability or possibility.

In many situations, both are natural, but lykkes often emphasizes success, while kan emphasizes can / be able to.

Igen means again.

So:

• prøver hun igen i morgen = she tries again tomorrow

Word by word:

• prøver = tries
• hun = she
• igen = again
• i morgen = tomorrow

Its placement here is normal and natural. Danish adverbs like igen often come after the subject in a main clause like this.

Yes, in standard Danish writing, a subordinate clause at the beginning is normally followed by a comma before the main clause.

So:

• Hvis det ikke lykkes hende at logge ind i aften, prøver hun igen i morgen.

That comma helps show where the if-clause ends and the main clause begins.