Upewnij się, że podkurzacz działa.

Upewnij się means make sure.

It comes from the verb upewnić się, which means to make sure / to become certain. The się is part of the verb, not a separate word with its own meaning here. In many Polish verbs, się marks a reflexive or fixed verb form.

So you should learn upewnić się as one whole expression.

In this sentence:

• upewnij = imperative form, meaning make sure
• się = part of the verb

Together: Upewnij się = Make sure

Because upewnij is the imperative form, used for giving a command or instruction.

The dictionary form is upewnić się.
When you want to tell one person make sure, Polish changes it to upewnij się.

Compare:

• upewnić się = to make sure
• upewnij się = make sure! (said to one person)

So this is the same kind of change as:

• English to check → check!
• Polish sprawdzić → sprawdź!

Że means that and introduces the second clause.

So:

• Upewnij się = make sure
• że podkurzacz działa = that the vacuum cleaner works

In English, that is often optional:

• Make sure the vacuum cleaner works
• Make sure that the vacuum cleaner works

In Polish, że is normally used in this structure, so it sounds natural and expected.

Działa is the 3rd person singular present tense of działać, meaning to work, to function, or to operate.

Here it means that the machine is functioning properly.

So:

• Telefon działa = The phone works
• Samochód działa = The car works
• Odkurzacz działa = The vacuum cleaner works

Polish uses the present tense here just like English does.

It is in the nominative case, because it is the subject of działa.

In other words, the vacuum cleaner is the thing doing the working/functioning.

Structure:

• podkurzacz = subject
• działa = verb

So the basic pattern is:

• X działa = X works

That is why the noun stays in nominative.

Probably not in this sentence. Most likely, this is a typo for odkurzacz, which is the normal Polish word for vacuum cleaner.

So the standard sentence would be:

Upewnij się, że odkurzacz działa.

A learner would definitely want to know this, because odkurzacz is a very common household word, while podkurzacz is not the standard word for this object.

That is normal Polish word order. The clause after że often keeps a straightforward subject + verb pattern:

• podkurzacz działa = the vacuum cleaner works

So:

• że podkurzacz działa = that the vacuum cleaner works

This is very natural in Polish.
You can sometimes change word order for emphasis, but the version here is the most neutral and standard.

Upewnij się is addressed to one person informally.

Polish distinguishes between informal singular, informal plural, and formal ways of speaking.

Here are some versions:

• Upewnij się, że odkurzacz działa. = informal, to one person
• Upewnijcie się, że odkurzacz działa. = informal, to more than one person
• Proszę się upewnić, że odkurzacz działa. = polite/formal

So the sentence you have is the singular informal command.

Sometimes się can move around in Polish, but in this sentence Upewnij się is the normal and natural order.

So this is the standard way to say it:

• Upewnij się, że odkurzacz działa.

For learners, it is best to memorize upewnić się and use the imperative as upewnij się.

Because działa covers the general idea of works / is functioning. In this kind of sentence, Polish does not need a separate progressive form like English is working.

So działa can mean:

• it works
• it is working
• it functions

The exact English translation depends on context, but the Polish form stays the same.

Here, the meaning is simply: check that the vacuum cleaner is functioning properly.