To jest chłopiec, który czeka przed szkołą.

To jest is a very common way to say this is or it is when identifying someone or something.

So:

• To jest chłopiec = This is a boy / It is a boy
• To here is a neuter demonstrative used in identification, not a masculine form matching chłopiec

In everyday speech, jest is often dropped:

• To chłopiec, który czeka przed szkołą.

That is also natural.

Chłopiec is in the nominative case, because it is the basic form used here after to jest in an identifying sentence.

Dictionary form:

• chłopiec = boy

In this sentence, chłopiec is the person being identified, so the nominative makes sense.

Here który means who or that in the sense of a relative pronoun:

• chłopiec, który czeka... = the boy who is waiting...

It connects chłopiec with the extra information czeka przed szkołą.

Because który must agree with the noun it refers to, and that noun is chłopiec.

• chłopiec is masculine singular
• so the relative pronoun is który

Compare:

• dziewczyna, która czeka = the girl who is waiting
• dziecko, które czeka = the child that is waiting

In Polish, relative clauses introduced by words like który are normally separated with a comma.

So:

• To jest chłopiec, który czeka przed szkołą.

This is standard Polish punctuation, even where English punctuation might feel different to a learner.

Czeka is the 3rd person singular present tense of czekać = to wait.

So:

• ja czekam = I wait / I am waiting
• ty czekasz = you wait
• on/ona/ono czeka = he/she/it waits / is waiting

Here it matches chłopiec, so:

• który czeka = who is waiting

It can mean either, depending on context.

Polish present tense often covers both:

• he waits
• he is waiting

So który czeka przed szkołą can naturally correspond to:

• who waits in front of the school
• who is waiting in front of the school

In this sentence, English would usually prefer who is waiting.

The preposition przed often requires the instrumental case when it means in front of in a location sense.

So:

• szkoła = nominative
• przed szkołą = in front of the school

This is why the ending changes from -a to -ą.

Not always, but very often when talking about location.

For example:

• przed domem = in front of the house
• przed szkołą = in front of the school

With some meanings involving time, przed can behave differently in translation:

• przed obiadem = before dinner

So for this sentence, the key thing to remember is:

• przed
  • instrumental = in front of / before

Polish has no articles like a and the.

So chłopiec can mean:

• a boy
• the boy

The exact meaning comes from context.

In this sentence, because the boy is being identified more specifically by the relative clause który czeka przed szkołą, English will often translate it as the boy who is waiting in front of the school, but Polish does not need a separate word for the.

Not in standard Polish for this sentence.

For people, standard Polish uses który:

• chłopiec, który czeka...

Using co here would sound nonstandard or dialectal. A learner should use który.

Yes, Polish word order is more flexible than English, but the original sentence is the most neutral and natural.

Standard:

• To jest chłopiec, który czeka przed szkołą.

You may also hear:

• To chłopiec, który czeka przed szkołą.

But for a learner, the original order is a very safe choice.

Roughly, chłopiec sounds like HWO-pyets, though that is only an approximation.

A few helpful points:

• chł sounds a bit like hw for many speakers
• o is like o in off (depending on accent)
• pie sounds roughly like pye
• c at the end sounds like ts

So:

• chłopiec ≈ HWO-pyets

Not exactly.

• To jest chłopiec = This is a boy / This is the boy depending on context
• To jest ten chłopiec = This is that boy / This is the boy with extra emphasis on a specific one

Ten makes it more explicitly definite and specific.

So in your sentence, plain To jest chłopiec, który czeka przed szkołą is perfectly natural.