Dok god si ovdje, osjećam se mirnije.

Dok god means as long as or so long as.

In this sentence, it introduces the idea for the whole time that this condition is true:

• Dok god si ovdje = As long as you are here

The word god adds emphasis. Without it, dok often means just while. With god, the phrase more clearly expresses duration with a conditional feel: for as long as this remains the case.

Si is the 2nd person singular present of biti (to be), so it means are:

• si = you are

Croatian usually does not need subject pronouns like ti because the verb form already shows who the subject is.

So:

• si ovdje = you are here

If you wanted extra emphasis, you could add ti:

• Dok god si ti ovdje...

That would sound more like As long as you are here... with emphasis on you.

This is the normal and natural Croatian word order.

A useful thing to know is that si is a clitic. Clitics are short unstressed words that usually cannot stand alone and tend to appear near the beginning of the clause.

So Croatian naturally gives:

• Dok god si ovdje

rather than something awkward like:

• Dok god ovdje si

For a learner, the safest thing is simply to remember dok god si ovdje as the standard pattern.

Because Dok god si ovdje is a subordinate clause placed before the main clause.

So Croatian punctuation works like English here:

• Dok god si ovdje, osjećam se mirnije.

The comma separates:

• the dependent part: Dok god si ovdje
• the main statement: osjećam se mirnije

Osjećam se means I feel.

The verb is osjećati se, which is the standard way to talk about how someone feels.

• osjećam = I feel
• se = a reflexive particle that belongs with the verb

Here se does not literally mean myself in the English sense. It is just part of the Croatian verb pattern.

Compare:

• Osjećam hladnoću. = I feel coldness / I feel the cold.
• Osjećam se umorno. = I feel tired.

So in your sentence:

• osjećam se mirnije = I feel calmer

Because se is also a clitic, and clitics in Croatian usually do not go in absolute first position.

So the normal order is:

• Osjećam se mirnije.

If another word comes first, se can follow that word:

• Ja se osjećam mirnije.

That is why both of these are possible:

• Osjećam se mirnije.
• Ja se osjećam mirnije.

But a sentence starting with just Se osjećam... would not be standard.

Mirnije is the comparative form, meaning more calmly, calmer, or more at peace.

So:

• mirno = calmly / calm
• mirnije = more calmly / calmer

In this sentence, the comparison is implied, not stated explicitly. The meaning is something like:

• I feel calmer than I otherwise do
• I feel calmer than when you are not here

This works just like English I feel better, where you do not have to say better than what.

So osjećam se mirnije is very natural Croatian for I feel calmer.

Yes, but the nuance changes slightly.

• Dok si ovdje = While you are here
• Dok god si ovdje = As long as you are here

So dok god sounds a bit stronger and more explicitly tied to duration/condition.

Your original sentence suggests:

• for the whole time that you are here, I feel calmer

If you remove god, the sentence is still correct, but it may sound a little less emphatic.

Often yes, but there is a slight difference in feel.

• ovdje = here
• tu = often there or right there, though in everyday speech it can also mean here

In many contexts, both are possible. But ovdje is the clearest, most straightforward choice for here in a sentence like this.

So:

• Dok god si ovdje... sounds very natural.

Using tu might also be understandable, but ovdje is the safer choice for learners.

It can do either, depending on context.

With the present tense, Croatian can express:

• a situation happening right now
• a general repeated feeling

So this sentence could mean either:

• Right now, as long as you are here, I feel calmer
• In general, whenever you are here, I feel calmer

The grammar allows both. The wider context tells you which meaning is intended.

It is contained in the verb form osjećam.

Croatian verb endings usually show the subject clearly:

• osjećam = I feel
• osjećaš = you feel
• osjeća = he/she feels

So Croatian does not need ja here unless you want emphasis:

• Ja se osjećam mirnije.

Without emphasis, the normal form is simply:

• osjećam se mirnije