Ten plecak jest za ciężki, żeby nieść go jedną ręką.

Because plecak is a masculine noun, and the demonstrative ten has to agree with it in gender, number, and case.

• ten = masculine singular
• ta = feminine singular
• to = neuter singular

So:

• ten plecak = this backpack
• ta torba = this bag
• to krzesło = this chair

The adjective later in the sentence also matches this masculine noun: ciężki.

Ciężki means heavy, and it has to agree with plecak.

Since plecak is:

• singular
• masculine
• nominative

the adjective also appears in the matching form:

• ciężki

Compare:

• ten plecak jest ciężki = this backpack is heavy
• ta torba jest ciężka = this bag is heavy
• to pudełko jest ciężkie = this box is heavy

So ciężki is simply the masculine singular nominative form.

Here za means too in the sense of excessively.

So:

• ciężki = heavy
• za ciężki = too heavy

This is a very common Polish pattern:

• za drogi = too expensive
• za mały = too small
• za trudny = too difficult

So Ten plecak jest za ciężki... means This backpack is too heavy...

In this sentence, żeby introduces a clause meaning to or in order to, but after za + adjective it often corresponds to English too ... to ...

So:

• za ciężki, żeby nieść go jedną ręką = too heavy to carry with one hand

Literally, Polish is closer to:

• too heavy, in order to carry it with one hand

But natural English uses too heavy to carry.

This is a useful pattern:

• Jest za zimno, żeby iść na spacer.
  = It’s too cold to go for a walk.
• Był za zmęczony, żeby pracować.
  = He was too tired to work.

This is an important aspect question.

Both verbs can relate to carrying, but they are not the same:

• nieść = to carry, to be carrying, to carry once/in one direction
• nosić = to carry habitually, repeatedly, or in general; also to wear

In this sentence, we mean carry this backpack in a specific situation, not carry backpacks in general, so nieść is the natural choice.

Compare:

• Nie mogę nieść tego plecaka.
  = I can’t carry this backpack.
• On często nosi ciężkie torby.
  = He often carries heavy bags.

So nieść fits the idea of carrying this particular backpack at this moment / in one act.

Go means it here, and it refers back to plecak.

Polish often includes an object pronoun in infinitive phrases like this:

• nieść go = to carry it

So the sentence literally includes the object of nieść: carry it.

Why go?

Because plecak is masculine singular, and here the pronoun is in the accusative/genitive-looking form used for masculine animate/inanimate? In this case, with nieść, the direct object form for a masculine inanimate noun is normally the same as nominative for the noun itself (plecak), but the pronoun is go.

Examples:

• Mam plecak. Widzę go.
  = I have a backpack. I see it.
• To jest mój telefon. Biorę go.
  = This is my phone. I’m taking it.

So go is simply the pronoun it.

Yes, you often can, especially if the meaning is clear:

• Ten plecak jest za ciężki, żeby nieść jedną ręką.

This can still be understood as This backpack is too heavy to carry with one hand.

But go makes the object explicit and often sounds more complete or natural, because the verb nieść normally has something being carried.

So:

• ...żeby nieść go jedną ręką = explicit: to carry it with one hand
• ...żeby nieść jedną ręką = understandable, but less explicit

Because after expressions like with one hand, Polish uses the instrumental case.

The noun ręka changes like this:

• nominative: ręka = hand
• instrumental: ręką = by hand / with a hand

And the adjective/number must match:

• jedna ręka = one hand
• jedną ręką = with one hand

So:

• jedną ręką = with one hand

This is a very common use of the instrumental in Polish:

• długopisem = with a pen
• nożem = with a knife
• lewą ręką = with the left hand

In Polish, in ordinary present-tense sentences like this, you usually keep the verb jest.

So:

• Ten plecak jest ciężki. = This backpack is heavy.

Unlike Russian, Polish normally does not drop the present tense of być in standard sentences of this type.

You may sometimes hear very informal omissions in headlines, notes, or special styles, but for normal speech and writing:

• jest is correct and expected.

You could also use aby, but żeby is much more common in everyday Polish.

So these are both possible:

• Ten plecak jest za ciężki, żeby nieść go jedną ręką.
• Ten plecak jest za ciężki, aby nieść go jedną ręką.

The second sounds a bit more formal or bookish.

In normal conversation, learners should usually prefer żeby.

The basic structure is:

• Ten plecak = subject
• jest = verb
• za ciężki = predicate adjective
• żeby nieść go jedną ręką = infinitive clause expressing the result/limit

So the sentence is very natural and neutral as written.

Polish word order is fairly flexible, but changing it often changes emphasis rather than basic meaning. For example, you might hear:

• Ten plecak jest za ciężki, żeby go nieść jedną ręką.

This is also very natural. In fact, many speakers may prefer żeby go nieść over żeby nieść go.

So the word order is not completely fixed, but the original sentence is correct and natural.

They are related, but not identical.

• jedną ręką = with one hand
• w jednej ręce = in one hand

In this sentence, jedną ręką is better because it describes the means by which you carry something.

• nieść go jedną ręką = carry it with one hand

If you say w jednej ręce, the focus is more on location/position:

• Trzymał torbę w jednej ręce.
  = He was holding the bag in one hand.

Sometimes both are possible depending on nuance, but here jedną ręką is the most natural choice.

A rough English-friendly guide:

• ciężki ≈ CHENSH-kee
• żeby ≈ ZHEH-bih

A few details:

• ci before a vowel-like sound gives a soft ch-like sound
• ę is a nasal vowel, though in normal speech it often changes depending on what follows
• ż sounds like the s in measure
• y is not exactly English ee; it is a harder central vowel

You do not need perfect pronunciation immediately, but it helps to notice that:

• ciężki is not pronounced like it is spelled in English
• żeby starts with a voiced zh sound

Not very naturally.

A more literal breakdown would be:

• Ten plecak = this backpack
• jest = is
• za ciężki = too heavy
• żeby = so that / to
• nieść go = carry it
• jedną ręką = with one hand

So a literal version might look like:

• This backpack is too heavy to carry it with one hand.

But in natural English, we usually do not keep it there:

• This backpack is too heavy to carry with one hand.

So the Polish sentence is natural Polish, but the most natural English translation is a little less literal.