Počítačová myš je na podložce pod myš.

A quick breakdown:

• počítačová myš = nominative singular
  It is the subject of the sentence: the thing that is somewhere.

• na podložce = locative singular
  After na with the meaning of location on, Czech uses the locative.

• pod myš = accusative singular
  This is part of the fixed expression podložka pod myš = mouse pad.

So the structure is basically:

[subject in nominative] + je + [location phrase]

Because Czech adjectives must agree with the noun they describe in gender, number, and case.

Here, myš is:

• feminine
• singular
• nominative

So the adjective also has to be:

• feminine
• singular
• nominative

That gives počítačová myš.

Compare:

• počítačový program = masculine
• počítačová myš = feminine
• počítačové hry = plural

Yes, myš by itself can mean computer mouse.

Czech uses the same word myš for:

• the animal mouse
• the computer device mouse

In everyday computer context, people often just say myš.
Počítačová myš is more explicit and is especially useful for learners or when you want to avoid ambiguity.

Because Czech distinguishes between:

• location = where something is
• direction/motion = where something goes

With na:

• na + locative = on something, in a location
• na + accusative = onto something, movement toward a surface

Here the mouse is already there, so it is location:

• Myš je na podložce. = The mouse is on the pad.

If you were moving it there, you would use the accusative:

• Dám myš na podložku. = I’ll put the mouse onto the pad.

Because after na in a location meaning, the noun goes into the locative singular.

The noun podložka is a feminine noun ending in -ka, and in the locative singular that often changes to -ce:

• podložka → na podložce
• taška → v tašce
• ruka → v ruce

So this is a normal declension pattern, not an irregular one.

Because in podložka pod myš, Czech uses a fixed expression with pod + accusative.

So here:

• myš is accusative singular
• it just happens to look the same as the nominative form

That is very common with some feminine nouns.

By contrast, pod myší would usually mean under the mouse in a literal location sense, using the instrumental after pod:

• Kabel je pod myší. = The cable is under the mouse.

So:

• podložka pod myš = mouse pad
• pod myší = under the mouse

They are different structures.

Because they do different jobs.

• na podložce tells you the actual location of the computer mouse: it is on the pad.
• pod myš is part of the noun phrase podložka pod myš, which means mouse pad.

Literally, podložka pod myš is something like:

• pad under a mouse
• or more naturally in English, mouse pad

So only the first prepositional phrase describes where the mouse is. The second one is part of the name of the object.

In a normal sentence like this, je should be there.

Czech often drops subject pronouns:

• Já jsem often becomes just jsem in context

But Czech normally does not drop the present-tense verb to be in ordinary affirmative sentences like this one.

So:

• Počítačová myš je na podložce pod myš. = normal full sentence

Without je, it would sound incomplete in standard Czech.

Czech has no articles like English the and a/an.

So the sentence could correspond to:

• The computer mouse is on the mouse pad
• or A computer mouse is on a mouse pad

Which one is meant depends on the context.

Czech usually expresses definiteness through:

• context
• word order
• stress
• sometimes demonstratives like ten, if needed

Not as fixed as in English.

This sentence has a very natural neutral order:

• Počítačová myš je na podložce pod myš.

But Czech can move parts around for emphasis, because case endings show the grammatical roles.

For example:

• Na podložce pod myš je počítačová myš.

This puts more focus on the location.

So the original order is normal, but not the only possible one.

A few helpful pronunciation points:

• č sounds like ch in church
• š sounds like sh
• ž sounds like the s in measure
• í is a long vowel

So roughly:

• počítačová ≈ po-chee-ta-cho-vá
• myš ≈ mish
• podložce has žc, pronounced roughly like zh-ts

Czech stress is usually on the first syllable of each word:

• POčítačová MYŠ je na PODložce pod MYŠ

That basic first-syllable stress rule is very reliable in Czech.