Kad govori istinu, njen pogled je miran.

Kad means when here. It introduces a time clause: Kad govori istinu = When she tells/speaks the truth.

In Croatian, kad and kada both mean when. Kad is just the shorter, very common form.

Both can work, but they are slightly different in feel.

• govoriti istinu = to speak/tell the truth, often in a more general or ongoing sense
• reći/kaže istinu = to say the truth / tell the truth, often referring to a specific statement

Because govori is imperfective, it fits well with a general condition or repeated situation: When she tells the truth, her expression is calm.

Govori is:

• 3rd person singular
• present tense
• from the verb govoriti = to speak

So govori means he/she speaks, he/she is speaking, or in this kind of sentence, she tells/speaks.

Croatian often does not need the subject pronoun, so there is no separate word for she in the first clause. The verb form already tells you it is he/she/it, and context supplies that it is she.

Because it is the direct object of the verb govori.

The basic form is istina = truth.
Here, after the verb, it becomes istinu, which is the accusative singular.

So:

• istina = nominative
• istinu = accusative

This is very common in Croatian: nouns change form depending on their role in the sentence.

Yes. Njen and njezin both mean her.

So both are possible:

• njen pogled je miran
• njezin pogled je miran

In standard Croatian, njezin is often preferred by some speakers and in more formal usage, but njen is also widely used and understood.

Pogled literally means look, gaze, or expression of the eyes.

In this sentence, it refers to the way her eyes/look appear. So njen pogled je miran means something like:

• her gaze is calm
• her look is calm
• her expression is calm

It does not mean viewpoint here, although pogled can also mean view or perspective in other contexts.

Because miran agrees with pogled, not with the woman.

• pogled is a masculine singular noun
• so the adjective must also be masculine singular: miran

If the noun were feminine, the adjective would change:

• njezina narav je mirna = her nature is calm

So even though we are talking about her, the adjective matches the noun pogled.

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

So:

• njen pogled je miran = her gaze is calm

Croatian often uses this verb just like English uses is.

Because Kad govori istinu is a subordinate clause placed before the main clause.

So the structure is:

• Kad govori istinu, = subordinate time clause
• njen pogled je miran. = main clause

This is similar to English:
When she tells the truth, her gaze is calm.

Yes, Croatian word order is fairly flexible.

For example, you could also say:

• Njen pogled je miran kad govori istinu.

This means the same thing. The difference is mostly in emphasis:

• starting with Kad govori istinu emphasizes the condition/time first
• starting with Njen pogled je miran emphasizes the main statement first

Most naturally, it sounds like a general observation or habitual truth:

• Whenever she tells the truth, her gaze is calm.

Because of kad + present tense and the imperfective verb govori, it usually suggests a repeated or typical situation rather than one single moment.

Usually yes in sentences like this, but depending on context it can also feel like whenever.

Here, a very natural English sense is:

• When she tells the truth, her gaze is calm
• or Whenever she tells the truth, her gaze is calm

So kad can cover both ideas, and context tells you which reading is more natural.