Ne lažite u obrascu.

Lažite is the 2nd person plural imperative of lagati, which means to lie.

So in this sentence it means:

• you all, lie in the positive
• with ne: do not lie

In Serbian, the 2nd person plural form is also used for polite/formal singular, so Ne lažite can mean either:

• Don’t lie when speaking to several people
• Don’t lie when speaking politely to one person

Ne is the normal negation word meaning not.

Here it turns the command into a negative command:

• Lažite. = Lie.
• Ne lažite. = Don’t lie.

So ne + imperative gives a prohibition or warning.

It can be either one, because Serbian uses the same verb form for both:

• plural you: talking to more than one person
• formal you: talking politely to one person

So without more context, Ne lažite could mean:

• Don’t lie, everyone
• Don’t lie, sir/ma’am

If you wanted the informal singular, you would say:

• Ne laži u obrascu.

Because the preposition u here means in / on / inside, and after u in this kind of meaning, Serbian uses the locative case.

So:

• base form: obrazac = form
• after u: u obrascu = in/on the form

Serbian often uses u where English might naturally say on a form, in a document, or on an application. The Serbian idea is more like inside the form / in the contents of the form.

Because obrazac is changing from its dictionary form to the locative singular.

The noun declines like this:

• obrazac = nominative singular
• obrasca = genitive singular
• obrascu = dative/locative singular

Two things are happening:

1. The ending changes to -u for the locative singular.
2. The a in the last part of obrazac disappears in the oblique forms.

So:

• obrazac
• u obrascu

This disappearing a is a common pattern in Serbian and is often called a mobile/fleeting a.

Yes, very often that is the most natural English translation.

Literally, u obrascu is in the form, but English usually says:

• on the form
• in the form
• on an application
• in the document

depending on context.

So if the meaning shown to the learner is Don’t lie on the form, that is a very natural translation even though Serbian uses u.

Yes. That is also natural Serbian.

A few possible ways to express this idea are:

• Ne lažite u obrascu.
• Nemojte da lažete u obrascu.
• Nemojte lagati u obrascu.

These are all ways to say Don’t lie on the form.

Very roughly:

• Ne lažite is short, direct, and common.
• Nemojte da lažete can sound a bit fuller or softer in some contexts.
• Nemojte lagati is also standard.

As a learner, it is good to recognize all of them.

The infinitive is lagati = to lie.

It is an imperfective verb. That makes sense here because the sentence is a general prohibition: do not lie.

In Serbian, aspect matters a lot, but for a learner the key point here is simply:

• lagati = the basic verb to lie
• lažite = imperative form built from that verb

A simple English-friendly approximation is:

• LA-zhee-teh

More specifically:

• ž sounds like the s in measure or the g in mirage
• te is pronounced clearly, not like English reduced tuh

So lažite sounds approximately like LA-zhee-teh.

It would be:

• Ne laži u obrascu.

Comparison:

• Ne laži = informal singular, talking to one person casually
• Ne lažite = plural or formal singular

So the original sentence uses the more polite or plural form.