Dzieci często mylą przecinek z kropką.

Because dzieci is the nominative plural form of dziecko (child). It is an irregular noun.

• singular: dziecko = child
• plural: dzieci = children

In this sentence, dzieci is the subject, so it appears in the nominative plural.

Mylą is the 3rd person plural present tense form of mylić.

So:

• ja mylę = I confuse / mix up
• ty mylisz = you confuse
• on/ona myli = he/she confuses
• my mylimy = we confuse
• wy mylicie = you (plural) confuse
• oni/one mylą = they confuse

Since dzieci means children, the verb has to be they form: mylą.

This is because the verb pattern is:

mylić [something] z [something]

It means to confuse [something] with [something].

So the two nouns take different cases:

• przecinek is the direct object, so it is in the accusative
• kropką comes after z, and in this meaning z requires the instrumental

That is why we get:

• przecinek = accusative singular
• kropką = instrumental singular

Because przecinek is a masculine inanimate noun. For many masculine inanimate nouns in Polish, the accusative singular is the same as the nominative singular.

So:

• nominative: przecinek
• accusative: przecinek

No visible change happens here.

Kropką is instrumental singular of kropka.

The forms are:

• nominative: kropka
• instrumental: kropką

After z in the pattern mylić X z Y, Polish uses the instrumental, so z kropką means literally with a period/full stop, but idiomatically the whole phrase means to confuse X with Y.

Yes, z often means with, and that is still the literal meaning here. But in Polish, the standard expression for to confuse A with B is:

mylić A z B

So even though English says confuse A with B, Polish uses exactly the same preposition idea here. What changes is the case after z: it takes the instrumental.

Często means often. It is an adverb describing how often the children do this.

The sentence order here is very natural:

Dzieci często mylą przecinek z kropką.

But Polish word order is flexible, so you could also hear things like:

• Dzieci mylą często przecinek z kropką.
• Często dzieci mylą przecinek z kropką.

These versions are possible, but the original sounds neutral and standard.

Mylić is imperfective. That matters because the sentence describes a habitual / repeated action:

Children often confuse...

For habits, general truths, and repeated actions, Polish normally uses the imperfective.

The perfective partner is usually pomylić. That would be used for a single completed mistake, for example:

• Dziecko pomyliło przecinek z kropką. = The child confused a comma with a period.

So in your sentence, mylić is the correct aspect.

Not for the same meaning. In Polish, a perfective verb in the present-looking form usually refers to the future.

So:

• mylą = they confuse / they are confusing / they often confuse
• pomyślą / pomylą-type perfective forms usually mean will confuse

For example:

• Dzieci pomylą przecinek z kropką. = The children will confuse a comma with a period.

So for a general statement with often, the imperfective mylą is the natural choice.

Because Polish does not have articles like English a/an and the.

So przecinek can mean:

• a comma
• the comma

and kropką can correspond to:

• with a period
• with the period

The exact interpretation comes from context, not from an article.

No. Mylić is a very common verb and can be used more broadly for mixing up or confusing people, things, names, dates, etc.

Examples:

• Myli imiona. = He/She mixes up names.
• Często mylę wtorek ze środą. = I often confuse Tuesday with Wednesday.
• Mylicie mnie z kimś innym. = You are mistaking me for someone else.

There is also mylić się, which means to be mistaken:

• Myliłem się. = I was mistaken.

A couple of pronunciation points are useful here:

• y in mylą is not like English ee. It is a deeper Polish vowel.
• ą is a nasal vowel, though at the end of a word it is often pronounced more like om/on depending on the speaker and context.
• kropką also ends in ą, so the same note applies there.
• ci in dzieci sounds like a soft ch-like consonant, not like English see.

A rough learner-friendly approximation might be:

• dzieci ≈ jyeh-chee
• mylą ≈ mih-won / mih-lon-like approximation
• kropką ≈ KROP-kon-like approximation

These are only rough guides, but they help you avoid reading the words as if they were English.