Všiml jsem si chyby.

The dictionary form is všimnout si.

A learner should treat si as part of the verb here, because všimnout si means to notice / to become aware of. If you leave out si, you no longer have the normal verb with this meaning.

Its imperfective partner is všímat si.

• všimnout si = perfective, a completed act of noticing
• všímat si = imperfective, noticing in general, paying attention, or repeated noticing

So in this sentence, Všiml jsem si chyby, the speaker is talking about one completed moment of noticing.

Si is a reflexive clitic, but here it is best understood as a fixed part of the verb všimnout si.

It does not literally mean to myself in the way an English speaker might expect. In this expression, it is just how Czech forms the verb.

So:

• všimnout si něčeho = to notice something
• všímat si něčeho = to notice / pay attention to something

You should learn the whole pattern together, including the case it requires.

Because všimnout si requires the genitive case.

The noun chyba is the basic dictionary form, in the nominative singular. But after všimnout si, it changes to the genitive singular:

• nominative: chyba = an error / mistake
• genitive: chyby

So:

• Všiml jsem si chyby. = I noticed a mistake.

This is one of the most important things to memorize with this verb:
všimnout si + genitive

Formally, chyby can be more than one thing in Czech, depending on context:

• genitive singular of chyba
• nominative plural of chyba
• accusative plural of chyba

But in this sentence, it is clearly genitive singular, because všimnout si requires the genitive.

So here chyby means of a mistake / a mistake, not mistakes.

This is standard Czech past tense word order.

The past tense is made from:

• a past participle: všiml
• an auxiliary clitic: jsem

In neutral word order, Czech often puts the participle first and the clitic second:

• Všiml jsem si chyby.

The clitic jsem usually likes to stand in the second position of the clause, not at the very beginning and not stressed.

So všiml jsem is the normal arrangement here.

Because the speaker is understood to be male.

In the Czech past tense, the participle agrees with the subject in gender and number.

• male speaker: Všiml jsem si chyby.
• female speaker: Všimla jsem si chyby.

So if a woman says I noticed a mistake, she would normally say:

• Všimla jsem si chyby.

Because Czech usually does not need subject pronouns unless you want emphasis or contrast.

The verb form already tells you the person:

• jsem = I am / I have as the 1st person singular auxiliary in the past tense

So Všiml jsem si chyby already clearly means I noticed a mistake.

You can add já if you want contrast or emphasis:

• Já jsem si chyby všiml. = I noticed the mistake (for example, but someone else didn’t)

But in a normal neutral sentence, leaving out já is more natural.

It is the past tense.

Czech past tense is formed with:

1. a past participle
2. the auxiliary být in the present tense

Here:

• všiml = past participle
• jsem = 1st person singular auxiliary

Together:

• Všiml jsem si = I noticed

Because the verb is perfective (všimnout si), the sentence naturally refers to a completed event in the past: one moment when the speaker noticed something.

This is a question of aspect, which is very important in Czech.

• všimnout si = perfective
  focuses on the completed act of noticing
• všímat si = imperfective
  focuses on the process, habit, or repeated noticing

Examples:

• Všiml jsem si chyby. = I noticed a mistake.
• Nevšímal jsem si chyb. = I wasn’t noticing mistakes / I didn’t pay attention to mistakes.

A simple way to remember it:

• všimnout si = to notice
• všímat si = to be noticing / to pay attention to

Yes, many Czech speakers do say všimnul jsem si, and you will definitely hear it in everyday speech.

But for learners, všiml jsem si is the safer and more standard form to produce, especially in careful or formal Czech.

So:

• všiml jsem si = standard, recommended for learners
• všimnul jsem si = common in colloquial speech

If you are aiming for textbook or exam Czech, use všiml jsem si.

It can change, because Czech word order is relatively flexible. But changing the order usually changes the focus or emphasis, not the basic meaning.

Neutral order:

• Všiml jsem si chyby.

Other possible orders:

• Chyby jsem si všiml.
  This puts more focus on the mistake
• Všiml jsem si té chyby.
  Adding té makes it that mistake / the mistake

So the sentence is not rigid, but the version you have is a very natural neutral one.

A very useful way to learn it is as a complete model:

všimnout si + genitive

Then your sentence is:

• Všiml = past participle, masculine singular
• jsem = I in the past tense auxiliary
• si = fixed reflexive part of the verb
• chyby = genitive singular of chyba

So the structure is:

Všiml jsem si chyby.
= I noticed a mistake.

If you memorize the whole pattern instead of translating word by word, this sentence becomes much easier to use correctly.