Nie głaszcz psa, kiedy je, bo wtedy woli być sam.

Because głaszcz is the imperative form, used for commands and instructions.

• głaszczysz = you pet / you are petting
• głaszcz = pet!
• nie głaszcz = don’t pet!

So Nie głaszcz psa is an instruction addressed to one person: Don’t pet the dog.

This is the singular informal command, used when speaking to one person:

• głaszcz! = pet!
• głaszczcie! = pet! (to more than one person)
• niech pan/pani głaszcze = a more polite form

Because psa is the accusative case of pies.

• pies = dog (dictionary form, nominative)
• psa = dog (accusative/genitive singular)

In this sentence, the dog is the direct object of głaszcz:

• głaszcz kogo? co? = pet whom? what?
• answer: psa

This is especially important because pies is a masculine animate noun. In Polish, masculine animate nouns usually take an accusative form that matches the genitive in the singular:

• widzę psa = I see the dog
• mam kota = I have a cat
• głaszcz psa = pet the dog

Polish often omits subject pronouns when they are clear from the verb form or context.

In this sentence:

• głaszcz already tells you the command is directed at you (singular informal)
• je means he/she/it eats, and here context tells us it means the dog eats

So Polish does not need to say:

• Ty nie głaszcz psa...
• ...kiedy on je...

Those pronouns are possible, but they are usually unnecessary and can sound emphatic.

It means when it is eating or when it eats.

• kiedy = when
• je = eats / is eating

The verb je comes from jeść = to eat.

Present tense in Polish often covers both:

• it eats
• it is eating

So kiedy je can naturally correspond to English when it’s eating in this context.

Yes, bo means because, and wtedy means then / at that time.

So:

• bo = because
• wtedy = then, at that moment

Together, bo wtedy woli być sam means something like: because at that time it prefers to be alone

In natural English, we often shorten that to: because it prefers to be alone then or simply because it prefers to be left alone while eating

The word wtedy adds the idea of specifically in that situation / at that moment.

Because after być (to be), Polish usually uses the nominative for the adjective describing the subject.

Here:

• woli = prefers
• być = to be
• sam = alone / by himself

So być sam means to be alone.

The adjective sam agrees with the understood subject, which is the dog:

• masculine singular: sam
• feminine singular: sama
• neuter singular: samo

Since pies is masculine, we get sam.

Yes, and that is exactly why it is used here.

sam can mean:

• alone
• by oneself
• by himself/herself/itself

So woli być sam means:

• he/it prefers to be alone
• he/it prefers to be by himself

For an animal, this is very natural Polish.

Also, sam is different from samotny:

• sam = alone, by oneself
• samotny = lonely

The sentence is not saying the dog feels lonely. It says the dog prefers to be left alone.

Polish usually prefers the imperfective in general negative commands.

• głaskać = to pet, to be petting, repeated/general action
• pogłaskać = to pet once / to give a petting stroke, perfective

So:

• Nie głaszcz psa, kiedy je = Don’t pet the dog when it’s eating
  This is a general rule or instruction.

Using Nie pogłaszcz psa would sound much less natural here. It would suggest something more like don’t give the dog a pet in one specific instance, rather than expressing a general habit/rule.

Yes. Polish word order is more flexible than English, although some orders sound more natural than others.

The given sentence:

• Nie głaszcz psa, kiedy je, bo wtedy woli być sam.

A different but still natural order could be:

• Kiedy je, nie głaszcz psa, bo wtedy woli być sam.

This puts the when-clause first and makes the timing especially clear.

The original version is very natural too. It starts with the main instruction first:

• Don’t pet the dog...

That is often a good choice when giving advice or warnings.

It can mean both.

In Polish, the 3rd person singular present tense form is often the same for:

• he
• she
• it

So je can mean:

• he eats
• she eats
• it eats

Here, because the subject is the dog, English naturally uses it:

• kiedy je = when it eats / when it’s eating

But grammatically, Polish does not need a separate verb form for it.

Literally, it is singular: the dog / a dog.

• psa = one dog
• je = it eats
• woli być sam = it prefers to be alone

But in context, Polish often uses the singular for a general instruction about a typical animal:

• Nie drażnij psa = Don’t tease a dog / the dog
• Nie głaszcz psa, kiedy je = Don’t pet a dog when it’s eating

So even though the grammar is singular, the meaning can be general advice rather than about one specific dog.