Szkło jest w zielonym pojemniku.

Because after w in the meaning of in for location, Polish uses the locative case.

So:

• dictionary form: zielony pojemnik = green container
• after w: w zielonym pojemniku = in the green container

Both the adjective and the noun have to change form:

• zielony → zielonym
• pojemnik → pojemniku

This is one of the most important patterns in Polish: prepositions often force a specific case.

Pojemniku is in the locative singular.

The basic form is pojemnik.
After w when you are talking about being inside a place, it becomes pojemniku.

So:

• pojemnik = container, bin
• w pojemniku = in the container

Because adjectives in Polish must agree with the noun in:

• gender
• number
• case

Here, pojemnik is:

• masculine
• singular
• locative

So the adjective must also be masculine singular locative:

• zielony pojemnik
• w zielonym pojemniku

You are seeing agreement in action.

Grammatically, yes in a full neutral sentence: Szkło jest w zielonym pojemniku.

But in everyday Polish, especially on labels, signs, or short instructions, jest is often omitted:

• Szkło w zielonym pojemniku

That sounds more like a label or heading, not a full descriptive sentence.

So:

• Szkło jest w zielonym pojemniku. = full sentence
• Szkło w zielonym pojemniku. = shortened, label-like style

Yes. Szkło is a singular neuter noun, but it often works like an uncountable or category noun, similar to English glass as a material.

In recycling language, szkło usually means glass waste / glass items as a category, not one specific piece of glass.

So even though the idea is general, Polish still uses the singular noun:

• szkło not
• szkła in this sentence

Because Polish has no articles.

English distinguishes:

• a green container
• the green container

Polish does not use separate words for a or the.
Context tells you whether something is general or specific.

So w zielonym pojemniku can mean:

• in a green container
• in the green container

depending on the situation.

Yes. Polish word order is more flexible than English because endings show grammatical relationships.

The sentence:

• Szkło jest w zielonym pojemniku.

could also be:

• W zielonym pojemniku jest szkło.

Both are correct, but the emphasis changes a bit.

• Szkło jest w zielonym pojemniku focuses first on glass
• W zielonym pojemniku jest szkło focuses first on the location

The version you were given is a normal, clear sentence.

Often yes, but not always. In this sentence, w means in or inside:

• w zielonym pojemniku = in the green container

But w can also mean at or during in other contexts:

• w szkole = at school
• w poniedziałek = on Monday

So you should learn w by meaning and by the case pattern it uses in each type of expression.

This is a very useful contrast:

• w zielonym pojemniku = in the green container
  This describes location.

• do zielonego pojemnika = into the green container
  This describes direction / movement toward the inside.

Examples:

• Szkło jest w zielonym pojemniku. = The glass is in the green container.
• Wrzuć szkło do zielonego pojemnika. = Throw the glass into the green container.

Notice that do takes a different case, so the forms change:

• zielonym pojemniku
• zielonego pojemnika

Yes, very often. Pojemnik means container, and in everyday Polish it is commonly used for recycling bins and sorting containers.

Depending on context, people might also say:

• kosz = bin, trash can
• kontener = large container

But pojemnik sounds very natural in recycling language, especially when talking about color-coded containers.

A few key points:

• sz sounds like English sh
• ł sounds like English w
• ó sounds like u
• rz usually sounds like a voiced zh sound, though that does not appear in this sentence

For this sentence:

• szkło sounds roughly like shkwo
• zielonym starts with a soft sound, roughly zye-
• pojemniku is roughly po-ye-MNEE-koo

A very rough English-style approximation of the whole sentence is:

• SHKWO yest f zye-LO-nim po-ye-MNEE-koo

That approximation is not perfect, but it helps with the main difficult sounds.

Yes, szkło is a neuter noun, and that matters in other sentences because adjectives and past tense forms may need to agree with it.

In this particular sentence, the adjective zielonym does not agree with szkło. It agrees with pojemniku, because it describes the container, not the glass.

So:

• szkło = neuter subject
• pojemniku = masculine locative noun
• zielonym matches pojemniku

That is an important thing to notice: adjectives agree with the noun they describe, not automatically with the subject of the sentence.