W moim piórniku jest mała temperówka i jedenaście kredek.

Because after the preposition w meaning in, Polish uses the locative case when talking about location.

So:

• piórnik → w piórniku
• mój → moim to match the noun

The basic form is mój piórnik = my pencil case, but after w it becomes w moim piórniku = in my pencil case.

Mój is a possessive word, and in Polish words like this change form to match the noun's:

• gender
• number
• case

Here, piórnik is:

• masculine
• singular
• in the locative after w

So mój changes to moim.

Compare:

• mój piórnik = my pencil case
• w moim piórniku = in my pencil case

This is a very common question.

In Polish, when you use a sentence like this to present what is somewhere—basically a there is/there are type of sentence—singular jest is very natural, especially when the list comes after the verb.

So:

• W moim piórniku jest mała temperówka i jedenaście kredek.

This sounds like In my pencil case, there is...

If you put the whole subject first, plural agreement becomes more expected:

• Mała temperówka i jedenaście kredek są w moim piórniku.

So the original sentence is not strange; it is a normal way to describe contents.

Because these are two different structures.

Mała temperówka means a small sharpener:

• temperówka is singular
• mała agrees with it in gender, number, and case

But jedenaście kredek means eleven crayons, and after numbers like 5 and above, Polish usually puts the counted noun in the genitive plural.

So:

• temperówka = one sharpener
• jedenaście kredek = eleven crayons

That is why the forms look different.

Because after jedenaście (eleven), Polish uses the genitive plural form of the noun.

The dictionary form is:

• kredka = crayon

Its genitive plural is:

• kredek

So:

• jedna kredka = one crayon
• dwie kredki = two crayons
• jedenaście kredek = eleven crayons

Kredki is not wrong in general—it is the nominative plural form—but it does not fit after jedenaście.

Here temperówka is in the nominative singular.

So the sentence contains:

• mała temperówka → nominative singular
• jedenaście kredek → a numeral phrase with the noun in genitive plural

This is normal in Polish. Numeral phrases often behave differently from ordinary noun phrases.

Because temperówka is a feminine noun.

In Polish, adjectives must agree with the noun they describe. So:

• mały = masculine
• mała = feminine
• małe = neuter

Since temperówka is feminine, you say:

• mała temperówka

Yes, absolutely.

That sentence means:

• I have a small sharpener and eleven crayons in my pencil case.

It is very close in meaning, but the focus is slightly different:

• W moim piórniku jest... = describes what is inside
• Mam... = emphasizes possession, I have

Also notice that after mam, mała temperówka changes to małą temperówkę because it becomes the direct object.

Yes. Polish word order is flexible.

The original sentence:

• W moim piórniku jest mała temperówka i jedenaście kredek

is a natural way to introduce the contents of the pencil case.

You could also say:

• Mała temperówka i jedenaście kredek są w moim piórniku.

That version puts more emphasis on the objects themselves.

So the meaning stays similar, but the focus changes.

Because Polish does not have articles.

So Polish does not normally separate:

• a small sharpener
• the small sharpener

with different little words the way English does.

Instead, Polish relies on context to show whether something is definite or indefinite.

Yes. In this sentence, piórnik is the normal Polish word for a pencil case or school pen-and-pencil case.

Also:

• temperówka = sharpener, usually for pencils or crayons
• kredka = crayon or coloring pencil, depending on context

So the vocabulary in the sentence is very natural for school-related objects.