Sigurna sam da je peron blizu.

Both are possible, but sigurna sam is the more natural, neutral Serbian word order here.

Serbian often leaves out subject pronouns like ja (I) when they are not needed, because the verb already shows the person:

• sam = I am

So sigurna sam literally looks like sure am, but it naturally means I am sure.

If you say ja sam sigurna, the ja adds emphasis, something like:

• I am sure
• As for me, I’m sure

Because the speaker is female.

Siguran / sigurna / sigurno changes to match gender and number:

• siguran = masculine singular
• sigurna = feminine singular
• sigurno = neuter singular

So:

• a man says Siguran sam...
• a woman says Sigurna sam...

Sam is the 1st person singular present tense of biti (to be).

So:

• sam = am

In sigurna sam, the structure is:

• sigurna = sure/certain
• sam = am

Together: I am sure.

Because Serbian usually drops personal pronouns when they are already clear from the verb form.

Here, sam already tells you the subject is I.

Compare:

• Sigurna sam = I am sure
• Sigurna si = you are sure
• Sigurna je = she is sure

So ja is unnecessary unless you want emphasis or contrast.

This is a subordinate clause introduced by da.

A very useful pattern in Serbian is:

• [main clause] + da + [subordinate clause]

Here:

• Sigurna sam = I am sure
• da = that
• je = is
• peron = platform
• blizu = near / close

So:

• Sigurna sam da je peron blizu.
• I am sure that the platform is nearby / close.

In Serbian, da often covers the role of English that in this kind of sentence.

Je is the short, unstressed form of is.
Jeste is the long form.

In ordinary sentences, Serbian usually prefers the short clitic form:

• da je peron blizu

The long form jeste is used more for emphasis, contrast, or clarity:

• da jeste blizu = that it really is close

So in your sentence, je is the normal choice.

Peron usually means a platform, especially at a train station or bus station.

It is a masculine noun:

• taj peron = that platform

In this sentence, it is the subject of the subordinate clause:

• peron je blizu = the platform is close / nearby

Blizu means near or close. In sentences like this, it often works as a predicate word, something like English nearby or close.

So:

• Peron je blizu. = The platform is nearby / close.

It can also be used with a noun:

• blizu stanice = near the station

In that use, it is followed by the genitive case:

• stanica → stanice

Because blizu can stand on its own and simply mean nearby.

So:

• Peron je blizu. can mean The platform is close or more naturally The platform is nearby.

If you want to specify what it is near, Serbian can add that:

• Peron je blizu stanice. = The platform is near the station.
• Peron je blizu izlaza. = The platform is near the exit.

Yes, Serbian word order is fairly flexible, but not completely free.

The neutral version is:

• Sigurna sam da je peron blizu.

You could also say:

• Da je peron blizu, sigurna sam.

That sounds more marked or stylistic, with focus on the clause that the platform is close.

One important point: short verb forms like sam and je are clitics, and they usually appear in second position in their clause. That strongly affects natural word order.

A male speaker would say:

• Siguran sam da je peron blizu.

The only change is:

• sigurna → siguran

Everything else stays the same.

A simple pronunciation guide would be:

• Sigurna sam da je peron blizu
• roughly: SEE-goor-nah sahm dah yeh PEH-ron BLEE-zoo

A few helpful notes:

• g is always a hard g as in go
• j is pronounced like English y
• u is like oo in food
• z is like z in zoo

Yes, very common.

You can use the same pattern with many adjectives and expressions:

• Sigurna sam da je u pravu. = I’m sure that she/he is right.
• Uveren sam da će doći. = I’m convinced that he/she will come.
• Drago mi je da si ovde. = I’m glad that you’re here.

So [expression] + da + clause is something you will see very often in Serbian.