Moja teczka leży pod biurkiem, bo jest za ciężka na półkę.

Because teczka is a feminine noun in Polish, and the possessive adjective my has to agree with the noun’s gender.

So:

• mój = masculine
• moja = feminine
• moje = neuter

Since teczka is feminine, you say moja teczka.

Teczka usually means something like:

• briefcase
• folder
• document case

The exact translation depends on context. In this sentence, it is some kind of bag or case that can be placed under a desk or on a shelf.

It is a feminine noun, which is important because other words in the sentence agree with it.

Polish often uses more specific verbs for where something is positioned.

• leżeć = to lie, to be lying
• stać = to stand
• wisieć = to hang

So teczka leży pod biurkiem means the briefcase/folder is lying under the desk.

If you used jest, it would only mean is, which is grammatically possible in some situations but less natural here. Polish often prefers the posture/location verb.

Because this sentence describes a location, not movement.

With pod:

• pod + instrumental = under something, in a fixed location
• pod + accusative = movement to under something

So:

• leży pod biurkiem = it is lying under the desk
• wkładam teczkę pod biurko = I am putting the briefcase under the desk

Since the briefcase is already there, Polish uses the instrumental case: biurkiem.

Because after pod in a static location meaning, Polish uses the instrumental case.

The noun is:

• nominative: biurko = desk
• instrumental: biurkiem = under/with the desk form

So:

• pod biurkiem = under the desk

This is a very common pattern:

• pod stołem = under the table
• przed domem = in front of the house
• między krzesłami = between the chairs

Because Polish often leaves out subject pronouns when they are obvious from context.

In English, you need it is too heavy.
In Polish, you can simply say jest za ciężka because the subject is already understood to be moja teczka.

Also, the adjective ciężka is feminine, which helps show that it refers back to teczka.

So Polish does not need an extra pronoun here.

Because ciężka has to agree with teczka, which is feminine singular.

Adjective forms here are:

• ciężki = masculine
• ciężka = feminine
• ciężkie = neuter

Since teczka is feminine, the correct form is za ciężka.

Za before an adjective usually means too.

So:

• ciężka = heavy
• za ciężka = too heavy

Other examples:

• za drogi = too expensive
• za zimno = too cold
• za duża = too big

So jest za ciężka means it is too heavy.

This is a very common learner question because na can take different cases.

Here, na półkę literally suggests onto the shelf or for placement on the shelf. The idea is:

• it is too heavy to go on the shelf
• it is too heavy for the shelf

That is why Polish uses na + accusative here:

• półka = shelf
• na półkę = onto the shelf / for the shelf

This construction is idiomatic in Polish. Even though English says for the shelf, Polish keeps the sense of intended placement onto the shelf.

Yes, that is the most natural interpretation.

Jest za ciężka na półkę suggests that the briefcase is too heavy to be put on the shelf, probably because the shelf is not suitable or strong enough, or because it is impractical to place it there.

So the sentence implies:

• it is under the desk
• because putting it on the shelf would not work

Yes. Bo means because and is very common in everyday Polish.

You could also say ponieważ, which also means because, but it sounds a bit more formal or written.

So:

• bo = common, conversational
• ponieważ = more formal

Both are correct:

• Moja teczka leży pod biurkiem, bo jest za ciężka na półkę.
• Moja teczka leży pod biurkiem, ponieważ jest za ciężka na półkę.

Not completely. Polish word order is more flexible than English, although some orders sound more natural than others.

The given version is very natural:

• Moja teczka leży pod biurkiem, bo jest za ciężka na półkę.

You could also move things around for emphasis, for example:

• Pod biurkiem leży moja teczka, bo jest za ciężka na półkę.

That puts more focus on under the desk.

However, the original sentence is the most neutral and natural choice for a learner to use.

Półkę is the accusative singular form of półka.

Forms:

• nominative: półka = shelf
• accusative: półkę

Because the phrase is na półkę, Polish uses the accusative here.

This is a useful pattern to remember:

• na stół = onto the table
• na półkę = onto the shelf
• na krzesło = onto the chair

Yes, completely natural.

In Polish, leżeć is used for many objects that are resting horizontally or simply located somewhere in a way that is thought of as lying.

So things like books, bags, papers, folders, and briefcases can all leżeć somewhere.

That is why teczka leży pod biurkiem sounds normal.