Davalac krvi ne sme da laže.

Davalac means donor or more literally giver in this kind of context.

It comes from the verb davati = to give.
The ending -lac is used to form a noun for a person who does something.

So:

• davati = to give
• davalac = giver / donor

In this sentence, davalac krvi means blood donor.

Because krvi is the genitive case of krv (blood).

Serbian often uses the pattern:

• noun + noun in genitive

to express something like X of Y.

So:

• davalac = donor
• krvi = of blood

Together: davalac krvi = blood donor

This is very common in Serbian:

• čaša vode = a glass of water
• boca vina = a bottle of wine
• davalac krvi = blood donor

Serbian does not have articles like a, an, or the.

So davalac krvi can mean:

• a blood donor
• the blood donor
• blood donors in a general sense, depending on context

In this sentence, it most naturally means something general like:

• A blood donor must not lie
• Blood donors must not lie

English needs an article or some other way to show that meaning, but Serbian does not.

Ne sme means must not, is not allowed to, or may not.

It comes from the verb smeti, which is used for permission or acceptability.

So:

• sme = is allowed to / may
• ne sme = is not allowed to / must not

In this sentence, ne sme da laže means:

• must not lie
• is not allowed to lie

This is stronger than just doesn’t want to lie or can’t lie.

No, not exactly.

• moći usually means can / to be able to
• smeti usually means may / to be allowed to / to dare

Compare:

• Ne može da laže. = He cannot lie. / He is unable to lie.
• Ne sme da laže. = He must not lie. / He is not allowed to lie.

So in your sentence, the idea is about permission or prohibition, not physical ability.

In modern Serbian, it is very common to use:

• a finite verb + da
  • present tense

instead of an infinitive.

So after verbs like smeti, you often get:

• sme da kaže = he may say
• ne sme da puši = he must not smoke
• ne sme da laže = he must not lie

Serbian does have infinitives, but this da + present structure is extremely common in everyday language.

Laže is the 3rd person singular present tense of lagati = to lie.

So:

• ja lažem = I lie
• ti lažeš = you lie
• on/ona laže = he/she lies

Because davalac krvi is singular, the verb is also singular:

• Davalac krvi ne sme da laže.

Because the sentence expresses a general rule or ongoing prohibition, not one single completed act.

Lagati is imperfective and means to lie in a general or repeated sense.

So ne sme da laže means:

• must not lie
• must not be lying
• must not tell lies

If Serbian wanted to focus on a single completed lie, a perfective verb might be used in another context, but here the imperfective is the natural choice for a general statement.

Yes, depending on context, da laže can sometimes be understood as either:

• to lie in general
• to be lying at that moment

But without extra context, the most natural reading is the general one:

• A blood donor must not lie

If the situation were specifically about the donor answering medical questions right now, English might translate it as:

• The donor mustn’t be lying

Serbian often leaves that distinction to context.

Yes, Serbian word order is fairly flexible.

The neutral order here is:

• Davalac krvi ne sme da laže.

This puts the topic first and sounds natural and standard.

But other orders are possible for emphasis, for example:

• Ne sme davalac krvi da laže.
• Da laže, davalac krvi ne sme.

These sound more marked or emphatic. For a learner, the original order is the best one to use.

Yes, it sounds fairly standard and somewhat formal or official, which fits medical language well.

You may also see:

• donor krvi

This uses the international word donor instead of the native-style davalac.

Both are understandable, but davalac krvi often sounds especially suitable in official, institutional, or medical contexts.

In the plural, it would be:

• Davaoci krvi ne smeju da lažu.

Breakdown:

• davalac → davaoci = donors
• ne sme → ne smeju = must not / are not allowed to
• krvi stays krvi

So:

• Davaoci krvi ne smeju da lažu. = Blood donors must not lie.

Yes. The negation is on sme, and that makes the whole action prohibited.

So:

• sme da laže = may lie / is allowed to lie
• ne sme da laže = may not lie / must not lie

This is the normal way to negate this kind of structure in Serbian.

Yes, that is also possible.

• ne sme da laže
• ne sme lagati

Both can mean must not lie.

However, ne sme da laže is especially common and natural in everyday modern Serbian.
For learners, the da + present version is very important to recognize and use.

It can sound like any of those, depending on context, but most naturally it sounds like a rule or requirement.

For example, in a medical setting, it could mean that a blood donor is required to answer honestly. So the tone is less about abstract morality and more about what is permitted or forbidden.

That nuance comes mainly from ne sme:

• not allowed to
• must not
• forbidden to