Do koktajlu używam blendera, a ciasto mieszam mikserem.

Because the preposition do requires the genitive case, so koktajl becomes koktajlu.

Here do koktajlu means something like:

• for the smoothie
• to make the smoothie
• for use in the smoothie

So it is not the direct object of the verb. It is a prepositional phrase telling us what the blender is being used for.

The verb używać takes the genitive case in standard Polish.

So:

• blender → nominative
• blendera → genitive

That is why you say:

• używam blendera = I use a blender

This is something learners usually just need to memorize with the verb:

• używać + genitive

For example:

• używam telefonu
• używam komputera
• używam noża

Because mikserem is in the instrumental case.

In this sentence, mikserem expresses the tool or means used to do the action:

• mieszam mikserem = I mix with a mixer

So the structure is different from używam blendera:

• używać + genitive → używam blendera
• a verb like mieszać can take an instrument in the instrumental → mieszam mikserem

This is a very common pattern in Polish when talking about the thing you do something with.

In Polish, the instrumental case often expresses with/by means of without needing a separate word.

So:

• mikserem already means with a mixer
• nożem = with a knife
• łyżką = with a spoon

English uses a preposition here, but Polish often uses just the case ending.

Because ciasto is the direct object of mieszam, and for a neuter noun like ciasto, the accusative singular is the same as the nominative singular.

So:

• nominative: ciasto
• accusative: ciasto

That is why the form does not change.

Also, in this kind of sentence, ciasto often means cake batter or dough, not only a finished cake.

They are similar appliances, but they are not exactly the same.

• blender: usually for blending liquids or making smooth mixtures, like smoothies or soups
• mikser: usually for mixing or beating, especially batter, cream, eggs, etc.

So in this sentence the pairing is natural:

• koktajl → blender
• ciasto → mikser

In Polish, a often means and, but with a slight contrast or comparison. It can feel like:

• and
• while
• whereas

So:

• Do koktajlu używam blendera, a ciasto mieszam mikserem.

has the sense of:

• For a smoothie I use a blender, and for cake batter I mix with a mixer
• I use a blender for a smoothie, whereas I mix batter with a mixer

If you used i, it would sound more like a simple list of two actions, with less contrast.

Polish often drops subject pronouns when they are clear from the verb ending.

Here:

• używam = I use
• mieszam = I mix

So ja is not necessary.

You could say:

• Ja do koktajlu używam blendera, a ciasto mieszam mikserem

but that would add emphasis, something like I use a blender...

They are both present tense, first person singular.

• używam = I use
• mieszam = I mix

In a sentence like this, the present tense usually expresses a general habit, typical method, or what I normally do, not necessarily something happening right now.

So it can mean:

• I use a blender for smoothies, and I mix batter with a mixer

They are imperfective forms.

That makes sense because the sentence describes a general repeated action or usual way of doing things, not a single completed event.

• używać = imperfective
• mieszać = imperfective

If you wanted to describe one completed action in the past or future, Polish would often use perfective verbs instead.

Yes, Polish word order is fairly flexible, but changing it changes the focus or emphasis.

The original sentence is neutral and natural:

• Do koktajlu używam blendera, a ciasto mieszam mikserem.

You could also say:

• Blendera używam do koktajlu, a ciasto mieszam mikserem.
• Ciasto mieszam mikserem, a do koktajlu używam blendera.

These are grammatically possible, but they sound more marked or emphasize different parts.

For learners, the original order is a good natural model.

Yes. That would also be correct, but it would create a different structure.

Compare:

• używam blendera = I use a blender
• mieszam mikserem = I mix with a mixer
• używam miksera = I use a mixer

So Polish can express the idea in more than one way:

• Do koktajlu używam blendera.
• Ciasto mieszam mikserem.
• Do ciasta używam miksera.

All of these are natural; they just use different grammar patterns.

Not exactly.

• do koktajlu usually means for the smoothie / for making the smoothie / as part of preparing the smoothie
• na koktajl often means for a smoothie in the sense of purpose or destination, but it can sound a bit different depending on context

In this sentence, do koktajlu is very natural because it refers to what the blender is used for.

So a learner should treat do koktajlu używam blendera as a normal and idiomatic way to say it.