Jsem si jistý.

Because Czech often drops subject pronouns when the verb already shows who the subject is.

• jsem = I am
• so já is not necessary

So:

• Jsem si jistý. = I’m sure.
• Já jsem si jistý. also works, but já adds emphasis, contrast, or emotional force.

For example:

• Jsem si jistý. = neutral
• Já jsem si jistý. = I am sure / I really am sure

Jsem is the 1st person singular present tense of být (to be).

So:

• jsem = I am

In this sentence, it links the subject (I) with the adjective phrase si jistý.

Si is the short form of the reflexive pronoun in the dative case.

In být si jistý, it is part of a fixed expression meaning to be sure / to be certain.

A very natural way to understand it is:

• být si jistý = to be sure
• literally, something like to be certain to oneself, though that literal wording is not how we would translate it into normal English

So in this sentence, si does not mean yourself or himself in the usual English sense. It is simply part of the Czech pattern used with this adjective.

Usually, if you mean I am sure / certain, Czech strongly prefers Jsem si jistý.

Without si, the sentence sounds unnatural or changes the feel of the meaning. The expression být si jistý is the standard idiom for being sure about something.

So for learners, the safest rule is:

• use jsem si jistý, not jsem jistý, when you mean I’m sure

Because it agrees with the speaker’s gender.

In Jsem si jistý, the adjective jistý is masculine singular, so this form is used by a male speaker.

Other forms:

• masculine: Jsem si jistý.
• feminine: Jsem si jistá.
• plural, mixed or masculine personal group: Jsme si jistí.
• plural, non-masculine-personal group: Jsme si jisté.

So if a woman is speaking, she would normally say:

• Jsem si jistá.

It is nominative, because it is a predicate adjective after být (to be).

In other words:

• jsem = verb to be
• jistý describes the subject
• predicate adjectives after být are normally in the nominative

So jistý matches the subject in:

• gender
• number
• case

There are two very common patterns.

1. With a clause:

• Jsem si jistý, že... = I’m sure that...

Example:

• Jsem si jistý, že přijde. = I’m sure that he’ll come.

2. With a noun or pronoun in the instrumental:

• být si jistý čím = to be sure of something

Examples:

• Jsem si tím jistý. = I’m sure of it.
• Nejsem si výsledkem jistý. = I’m not sure about the result.

So a very useful thing to remember is that být si jistý can be followed either by:

• že
  • a clause
• or the instrumental case

Jsem si jistý is a completely normal, neutral word order.

The little word si is a clitic, which means it tends to appear near the beginning of the sentence, usually in second position.

So:

• Jsem si jistý. = normal and natural

Other word orders are possible, but they usually sound marked or emphasize something:

• Já si jsem jistý. = unnatural in normal speech
• Jistý si jsem. = strongly marked, poetic, or contrastive
• Jistý si nejsem. = more natural as a contrastive negative phrase: Sure, I am not.

For beginners, it is best to stick with:

• Jsem si jistý.

You negate the verb:

• Nejsem si jistý. = I’m not sure.

Here:

• jsem → nejsem

This is the standard and most common negative form.

It is neutral. You can use it in both everyday conversation and more formal contexts.

It is not slang, and it is not especially bookish either. It is just a standard Czech way to say I’m sure or I’m certain.

They are related in meaning, but they are not the same kind of expression.

• Jsem si jistý. = I’m sure.
  This is a full sentence about your state of certainty.
• Určitě. = Definitely / certainly / for sure.
  This is usually an adverb used as a reply or modifier.

Examples:

• Přijde? — Určitě. = Will he come? — Definitely.
• Jsem si jistý, že přijde. = I’m sure he’ll come.

So určitě often works like an adverbial reply, while jsem si jistý is a full predicative construction.

A rough English-friendly guide is:

• jsem ≈ ysem
• si = si
• jistý ≈ yis-tee

A more careful approximation would be:

• Jsem si jistý ≈ ysem si yist-ee

A few useful notes:

• Czech j is pronounced like English y in yes
• i is a short i
• ý is a long ee sound

In normal speech, jsem may sound a bit reduced, but learners should first aim for the clear form.

Yes, depending on context, Jsem si jistý can sometimes be translated as:

• I’m sure
• I’m certain
• I’m confident

But the most direct and common translation is I’m sure.

If the context is about confidence in a fact or conclusion, I’m sure or I’m certain is usually best. If the context is about confidence in one’s own ability, Czech may use other expressions too, depending on nuance.