Learn Hardcore Polish: Klawiatura mojego komputera jest głośna, ale mysz jest bardzo cicha. - The keyboard of my computer is loud, but the mouse is very quiet.

The structure is literally: Klawiatura (czyja?) mojego komputera = Keyboard (whose?) of my computer.
In Polish, when one noun belongs to another, the “owner” is put into the genitive:
- klawiatura telefonu – the keyboard of the phone
- klawiatura mojego komputera – the keyboard of my computer

The possessive pronoun mój (my) has to agree in case and gender with komputer, so it becomes:

Nominative: mój komputer – my computer (as subject)
Genitive: mojego komputera – of my computer

Here, because we need “of my computer”, we use genitive, so it must be mojego komputera, not mój komputer.

Why is it komputera and not just komputer?

Komputera is the genitive singular form of komputer.

You use the genitive:

to show possession (like English of or ’s)
after another noun that “owns” something

Patterns:

klawiatura komputera – the keyboard of the computer
klawiatura mojego komputera – the keyboard of my computer

So komputer (nominative) → komputera (genitive) because something belongs to it.

Why are the adjectives głośna and cicha, not głośny and cichy?

They have to agree with the gender and number of the nouns they describe.

klawiatura (keyboard) is feminine → adjective must be feminine:
- głośna klawiatura – a loud keyboard
mysz (mouse) is also feminine → adjective must be feminine:
- cicha mysz – a quiet mouse

Dictionary forms are:

głośny – loud (masculine basic form)
cichy – quiet (masculine basic form)

But here we need:

feminine singular nominative: głośna, cicha

So:
Klawiatura mojego komputera jest głośna, ale mysz jest bardzo cicha.
Both głośna and cicha are feminine singular nominative forms, matching klawiatura and mysz.

Are klawiatura and mysz always feminine? How am I supposed to know their gender?

Yes, klawiatura and mysz are both feminine nouns.

Some quick guidelines (with exceptions):

Most nouns ending in -a are feminine:
- klawiatura, kawa, książka, mapa
Many nouns ending in a consonant are masculine, but there are important feminine exceptions, including:
- mysz, twarz, noc, sól

You usually learn the gender with each noun, but patterns help. Dictionary entries often mark gender:

klawiatura (f)
mysz (f)

Because both are feminine, adjectives describing them use feminine forms (głośna, cicha).

Do I really need to repeat jest? Can I say: Klawiatura mojego komputera jest głośna, a mysz bardzo cicha?

You don’t have to repeat jest; both versions are correct:

With repetition (more explicit, very clear):
- Klawiatura mojego komputera jest głośna, ale mysz jest bardzo cicha.
Without repetition (a bit more compact, very natural):
- Klawiatura mojego komputera jest głośna, a mysz bardzo cicha.

In the second sentence, jest is simply understood before bardzo cicha.

Note: It’s more natural to use a instead of ale when you drop the second jest, because a works well for simple contrast between two parallel things: keyboard vs mouse. But ale is still grammatically possible.

What is the difference between ale and a here?

Both ale and a can be translated as but, but they’re used a bit differently:

ale – a stronger but, marks clearer opposition or contradiction.
- Jest drogo, ale ładnie. – It’s expensive, but (still) nice.
a – a milder but/and, often just contrasts or compares two things, sometimes closer to and:
- Ja wolę kawę, a on herbatę. – I prefer coffee, and he prefers tea.
- Klawiatura jest głośna, a mysz cicha. – The keyboard is loud, and the mouse is quiet.

In your sentence:

ale highlights the contrast more strongly:
- …jest głośna, ale mysz jest bardzo cicha.
a would sound slightly softer and more neutral:
- …jest głośna, a mysz jest bardzo cicha.

Both are acceptable; choice is stylistic.

Why is it bardzo cicha and not cicha bardzo?

In Polish, the usual, neutral word order for adverb + adjective is:

adverb – adjective – noun (or verb)

So:

bardzo cicha mysz – a very quiet mouse
mysz jest bardzo cicha – the mouse is very quiet

Putting bardzo after the adjective (cicha bardzo) is possible in speech for emphasis, but it sounds marked or poetic, not standard neutral word order. For everyday language, bardzo cicha is the natural choice.

Why do we use an adjective with jest here (jest głośna / jest cicha) and not an adverb like in some English patterns?

Polish, like formal English, uses adjectives after “to be” to describe a noun’s quality:

Klawiatura jest głośna. – The keyboard is loud.
(adjective, describing klawiatura)

When you describe how an action is done, you use an adverb:

Klawiatura stuka głośno. – The keyboard clacks loudly.
(adverb głośno, describing the verb stuka – clacks)

So:

jest głośna / cicha – adjective, describes what the keyboard/mouse is
gra głośno / cicho – adverb, describes how something plays/sounds

Sometimes I see jest + instrumental case (e.g. jest lekarzem). Why is it jest głośna, not jest głośną?

Polish uses two main patterns with jest:

jest + adjective (nominative) – to describe qualities:
- Klawiatura jest głośna. – The keyboard is loud.
- Mysz jest cicha. – The mouse is quiet.
- On jest zmęczony. – He is tired.
jest + noun (instrumental) – to say what someone/something is (profession, role, function, classification):
- On jest lekarzem. – He is a doctor.
- To jest komputerem? – Is this a computer? (a bit awkward, but grammatically fits the pattern)
- Ona jest nauczycielką. – She is a teacher.

Here, głośna and cicha are adjectives, so they stay in nominative feminine singular, not instrumental:

Correct: jest głośna, jest cicha
Wrong: jest głośną, jest cichą (that would suggest they are some kind of “loud thing” / “quiet thing” as a role, which is not how it works)

Why is there no word for “the” in klawiatura mojego komputera or mysz?

Polish has no articles (no “a/an/the”). Definiteness is usually understood from context.

So:

klawiatura mojego komputera can mean:
- the keyboard of my computer
- my computer’s keyboard
mysz can mean:
- a mouse
- the mouse

If you really need to emphasize something like this/that keyboard, you use demonstratives instead of articles:

ta klawiatura – this keyboard
tamta mysz – that mouse

Why is it mysz, not myszka? I often see myszka for “computer mouse”.

Both exist, but they feel a bit different:

mysz – literally mouse (animal), and also commonly used for a computer mouse, especially in more neutral or technical contexts.
myszka – a diminutive form (little mouse), very common in everyday speech for a computer mouse, and sometimes used affectionately for the animal too.

In your sentence:

…ale mysz jest bardzo cicha. – perfectly correct, neutral.
…ale myszka jest bardzo cicha. – also correct; sounds a bit more informal / cozy / colloquial.

Both are understood as “mouse” here, and context (computers) makes it obvious you mean the device, not the animal.