Liječnica tvrdi da je lijek siguran.

Because liječnica is the feminine form meaning female doctor, while liječnik is the masculine form meaning male doctor.

• liječnik = male doctor / doctor (masculine)
• liječnica = female doctor

So this sentence specifically tells you the doctor is a woman.

Tvrdi is the 3rd person singular present tense of tvrditi.

So it means:

• he/she/it claims
• he/she/it asserts
• sometimes says in a stronger, more definite sense

In this sentence, liječnica tvrdi = the doctor claims.

Da here means that and introduces a subordinate clause.

So:

• Liječnica tvrdi = The doctor claims
• da je lijek siguran = that the medicine is safe

This is a very common Croatian pattern:

• mislim da... = I think that...
• znam da... = I know that...
• kaže da... = he/she says that...
• tvrdi da... = he/she claims that...

Je is the 3rd person singular present of biti (to be).

So in da je lijek siguran, the structure is literally:

• da = that
• je = is
• lijek = the medicine
• siguran = safe

In other words: that the medicine is safe.

Croatian often uses short forms of biti like this:

• sam = am
• si = are
• je = is
• smo = are
• ste = are
• su = are

Because siguran agrees with lijek.

• lijek is a masculine singular noun
• adjectives in Croatian must agree with the noun in gender, number, and usually case

So:

• lijek → masculine singular
• siguran → masculine singular

Compare:

• lijek je siguran = the medicine is safe
• tableta je sigurna = the tablet is safe
• vino je sigurno = the wine is safe

Both liječnica and lijek are in the nominative singular.

Why?

• liječnica is the subject of the main clause: The doctor claims
• lijek is the subject of the subordinate clause: the medicine is safe

So the sentence contains two clauses, and each clause has its own subject:

1. Liječnica tvrdi
2. da je lijek siguran

That is why both nouns are nominative.

Because Croatian has no articles.

English distinguishes:

• a doctor
• the doctor
• a medicine
• the medicine

Croatian does not have separate words for a/an/the. So liječnica can mean:

• a doctor
• the doctor

and lijek can mean:

• a medicine
• the medicine

The exact meaning depends on context.

The given order is the most neutral and natural one:

Liječnica tvrdi da je lijek siguran.

Croatian word order is more flexible than English, but changing it usually changes emphasis or style, not the basic meaning.

For example, you may also hear:

• Liječnica tvrdi da je siguran lijek.
  This is less neutral and can sound marked or stylistically unusual in everyday speech.

• Lijek je siguran, tvrdi liječnica.
  This puts more focus on the medicine is safe.

So yes, word order can change, but the original sentence is the standard choice.

Yes. Croatian often omits subject pronouns because the verb form already shows the person.

So instead of saying:

• Ona tvrdi... = She claims...

Croatian very often just says:

• Tvrdi... = (She) claims...

In this sentence, the noun liječnica is already present, so there is no need for ona.

Adding ona would usually give extra emphasis or contrast:

• Ona tvrdi da je lijek siguran. = She claims that the medicine is safe.

Sometimes yes, but tvrdi is usually stronger than plain says.

Roughly:

• kaže = says
• tvrdi = claims / maintains / asserts

So tvrdi can suggest that the speaker is reporting someone’s statement in a more formal or possibly more skeptical way.

For example:

• Liječnica kaže da je lijek siguran. = The doctor says the medicine is safe.
• Liječnica tvrdi da je lijek siguran. = The doctor claims/asserts the medicine is safe.

It can mean both, depending on context.

With lijek, siguran means safe:

• Lijek je siguran. = The medicine is safe.

With a person, it can mean sure/certain:

• Sigurna sam. = I am sure.
• Jesi li siguran? = Are you sure?

So siguran is a flexible adjective, and context tells you which meaning is intended.

It is:

[main clause] + [da-clause]

More specifically:

• Liječnica = subject
• tvrdi = verb
• da = that
• je = is
• lijek = subject of the subordinate clause
• siguran = predicate adjective

So the structure is:

The doctor claims + that + the medicine is safe

This is a very common pattern in Croatian and worth learning early.