Kad je signal slab, ni korisničko ime ni lozinka ne pomažu.

Kad means when here. Kada is the longer form, and both are very common.

In this sentence, Kad je signal slab... means When the signal is weak...

In everyday speech, kad is often more common than kada. In most contexts, you can treat them as interchangeable.

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

So after Kad, the clitic je comes immediately:

• Kad je signal slab

A literal word-by-word logic is not very helpful here. The important thing to learn is:

• Croatian often places short unstressed words like je, sam, si, se, ga, mu near the beginning of the clause
• after a word like kad, they often appear right after it

So Kad je signal slab is the natural Croatian order.

Because slab has to agree with signal.

• signal is masculine singular
• so the adjective must also be masculine singular
• therefore: slab

Compare:

• signal je slab = the signal is weak
• veza je slaba = the connection is weak
• svjetlo je slabo = the light is weak

The ending changes depending on the gender and number of the noun.

Ni ... ni means neither ... nor.

So:

• ni korisničko ime ni lozinka = neither the username nor the password

You repeat ni before each item:

• ni A ni B
• ni kiša ni snijeg
• ni danas ni sutra

That repetition is normal and required.

Because Croatian normally uses negative concord. That means multiple negative elements appear together in the same sentence.

So with ni ... ni, the verb is also negative:

• ni korisničko ime ni lozinka ne pomažu

This is completely normal in Croatian.

In English, you usually avoid this and say:

• Neither the username nor the password helps

But Croatian prefers:

• ni ... ni
  • negative verb

So the extra ne is not a mistake; it is the standard pattern.

Because the subject has two parts:

• korisničko ime
• lozinka

Together they form a coordinated subject, so Croatian normally uses a plural verb:

• ni korisničko ime ni lozinka ne pomažu

That is why the verb is:

• pomažu = help / are of help

If there were only one subject, you would use singular:

• Lozinka ne pomaže
• Korisničko ime ne pomaže

They are in the nominative, because they are the subject of the verb.

You can see that in the basic dictionary forms:

• korisničko ime = username
• lozinka = password

Since they are the things that do not help, nominative is the correct case.

Because it is made of:

• korisničko = user-related
• ime = name

So literally it is something like user name, which matches English username.

Grammatically:

• ime is the noun
• korisničko is an adjective modifying it

They must agree:

• ime is neuter singular
• so korisničko is also neuter singular

This is a normal Croatian adjective + noun phrase.

Pomažu is the 3rd person plural present form of pomagati or, in practical terms here, to help / to be of help.

In this sentence, it does not mean physical help. It means:

• they are not useful
• they do not solve the problem
• they do not help in this situation

So the idea is:

• if the signal is weak, having the correct username and password will not solve anything

The present tense is used here to express a general fact or general situation.

So:

• Kad je signal slab, ni korisničko ime ni lozinka ne pomažu.

means something like:

• Whenever the signal is weak, neither the username nor the password helps.

Croatian often uses the present tense for general truths, repeated situations, and instructions.

Yes, you could, but the nuance changes slightly.

• Kad je signal slab = when the signal is weak / whenever the signal is weak
• Ako je signal slab = if the signal is weak

Kad sounds more like a situation that occurs and has this result. Ako sounds more explicitly conditional.

In many practical contexts, both would be understandable. In this sentence, kad sounds very natural because it describes what happens in that situation in general.